Hi there, bear18 ! Welcome to the forum
That's a nice room size for a "man cave" cozy and manageable, but large enough that you can comfortably fit a small group of friends.
When it comes to setting up a home theater system in a multi-purpose room like this, it really helps to plan things out in steps at a time. It just makes it easier to keep your thoughts and plans organized.
So the first things to do are to decide how the room is going to be divided and where the theater seats and TV are going to go. In terms of acoustics, it's best if you could put the TV on one of the 12 foot walls and have a couch or three theater seats about 7 feet away from that screen. That basically divides the room in half, with about a 2.5 foot walkway behind the seats, about a 2.5 foot walkway on either side of the couch or theater chairs (you really would NOT want to try and squeeze any more seats in going across the 12 foot room width), and then you'd have a 10 foot by 12 foot area in the back of the room where you can put your poker table and dart board.
The other option is more of a side-by-side setup, but that is less preferable just in terms of room acoustics, as one side of the theater would be close to a wall, while the other side of the theater would be completely open to the other half of the room. Also, a side-by-side setup would make it a lot harder to see the TV from the poker table, while a front/back setup gives the back half of the room a clear view of the TV still and the speakers will be firing back into that rear portion of the room. The front/back setup also allows you to easily bring more seats in and put them behind the main couch or 3 theater seats so that more people can enjoy a movie. The side-by-side setup would limit you a lot more, and I imagine you'll want to have more than just 3 seats available.
With only a 40" screen size, the "ideal" SMPTE seating distance would be a mere 5.42 feet! But that's unreasonably close, IMO, unless you're talking about a dorm room or a bedroom maybe. It barely gives you room to recline and put your feet up! A 7 foot viewing distance gives you just under a 24 degree field of view, which is basically the minimum field of view you'd ever want to use with an HD display. Move any farther away and you're basically "wasting" the HD resolution, since you wouldn't really be able to see any additional detail vs. an SD display from a greater distance.
Of course, you could always decide to upgrade your TV screen size in the future
You didn't specifically state your budget. That's a very important factor. But just going by the products you listed, I'm guessing you're willing to spend somewhere in the range of about $600 - $1000 ?
I don't have as big of a problem with buying a HTiB as a lot of other folks. Especially a fairly decent package like the ones offered by Onkyo, Denon and Yamaha. That said, you are definitely not getting the best performance possible with any of the "all-in-one" systems. The subwoofers in particular are a big step down from a "real" sub. I would also very much recommend that you avoid the Samsung HTiB packages, which are much more about style and extremely skinny speakers at the expense of actual good sound!
So, if you're willing to go towards the top end of your budget, you can get a much better sound for both movies and music.
By FAR, the most important parts are the front 3 speakers (Left, Center, Right) and the subwoofer. You can skimp quite a bit on the surround speakers without much detriment at all to the sound. Surround effects are meant to be ambient, not terribly directional, and to just envelop the audience, which is why the surround speakers are meant to be placed up high (2-3 feet above your head when you're sitting), directly to either side of the main row of seats or just slightly behind that row, and firing across the width of the room (ie. the surround speakers are pointed at each other and not aimed toward the audience). This placement most accurately recreates the surround speakers that you find in a movie theater, which are all up high, on the side walls, and firing across the theater's width
So a package I would highly recommend is the HSU Value 2 - 3.1 Package , which gives you a pair of HSU's very good HB-1 MK2 bookshelf speakers for Front L/R duty, the matching HC-1 MK2 Center speaker and the STF-2 subwoofer, which is a GREAT little sub for the price. The STF-2, while it can't quite reach right down to the lowest 20Hz notes, is about as good as it gets at this sort of price point. You need to spend at least $550 on the subwoofer alone if you want to start digging right down to 20Hz while keeping tight control for music and nice, linear, accurate output. The STF-2 gives up a little bit of that lowest extension, but what makes it so great is that it is very accurate, linear and tightly controlled. You said you wanted great sound for movies, but also for music. Musical performance is the big differentiator that makes the STF-2 so worth while for your setup, IMO
That's an $800 package. You can throw in some speakers that you might already own for surrounds, or just rustle up any pair of inexpensive speakers for surround duty.
To drive it all, I'm personally not in favor of ever buying a receiver that does not offer pre-outs for all channels. Pre-outs give you the option to connect a separate amp if you ever need more power. You won't find pre-outs for all of the speaker channels on most low cost receivers.
Luckily, the excellent Onkyo TX-NR709 can be had for a mere $430! These are fully authorized factory refurbished units, but don't be put off by that "refurbished" title. They still come with a full 1 year warranty - not the typical 90 day warranty on refurbished products - and that's because these are basically "clearance" items since the new TX-NR717 has come out to replace the 709 model from 2011. The funny thing is, the 709 is actually better than the newer 717!
So the 709 is a steal and by FAR the least expensive option for getting full 7.1 speaker pre-outs. It also offers Audyssey MultEQ XT auto speaker setup and room EQ. MultEQ XT is Audyssey's second from the top level of auto-calibration. Only MultEQ XT32 is higher. So this is quite a high value as the receiver can setup, calibrate and EQ even the tough bass frequencies, which is where you need EQ the most! Just an excellent value.
So if you go with these suggestions, you're looking at around $1300 or so once you factor in getting some inexpensive speakers for the surrounds as well as all of your speaker wire, cables and possibly some speaker stands or wall mounts. That might be a little more than you were planning to spend, but I can promise you that the few extra hundred dollars are WELL worth it. The Onkyo TX-NR709 receiver is an unmatched value and just an order of magnitude better in every way as compared to any of the HTiB or all-in-one receiver options. The HSU STF-2 subwoofer is a MASSIVE step up over any HTiB sub, and truly, one of the most important parts of any theater. You really do not want to skimp on your subwoofer, and the STF-2 is as good as it gets for so little money.
The HSU HB-1 MK2 & HC-1 MK2 is by no means the only good option. But they are very good speakers - especially at this sort of price point. What I think is very important is that they can really handle playing quite loud while staying very clean and clear, and they are very efficient, so the 709 will have no trouble at all making them absolutely sing. Your room size is not large, but if you go with my recommended front/back setup, you'll need so decent output if you want to play music while you're in the back of the room playing poker or darts. The HSU speakers can handle that without strain, which cannot be said for most speakers at this price.
Anywho, that's just one option, but it's the way I would spend my own money if I were in your situation with what I am guessing is close to your budget Head on over to monoprice.com to get all of your speaker wire, cables and mounts at the lowest prices you'll find anywhere. Crazy low prices really, but this is the ONE case where they're actually NOT too good to be true. Monoprice really is that inexpensive, but the products are still great and function perfectly
Hope that helps!