What's the ceiling height? I'm going to guess between 8 and 10 feet for now. Within that range, you've got between 3000 and 4000 cubic feet of volume. So a "large" room by THX standards.
The ceiling is 8ft, so I am sitting at about 3,100 cubic feet.
If you like to listen loud - as in close to the 85dB average with 115dB peaks in the LFE as specified as "reference level" - the a sub like the Rythmik FV12 just isn't going to cut it. As much as I love that sub and think it's the best $500 sub at the moment, it just doesn't offer quite that much output. Might be enough if you don't listen that loud though.
I'd highly recommend going with a pair of subs in your room. You've got a lot of seating area, and this is a dedicated room where you should be able to place the two subs well enough to create nice, even response without too many huge peaks or nulls at the seat. 4 subs would be nice, but there's the budget to consider
I did not even consider going 2 smallers subs vs the 1 larger one. Would 2 12's still be better than say the FV15HP from Rythmik? I kind of wanted to go with them as they are only 3 blocks away from me, have offered me a good price and will allow me to take one home and try it out for a few days before I commit.
With a 24 foot room length and two rows of seats, I'm going to guess that your primary seat is in the first row and about 16 feet or so from the front wall? Your 120 inch screen size would be perfect for that distance. I'm guessing that's not an accident
Yes, my front center seat is primary and the measurement from that seat to the front of my screen is 15'" currently. Funny you said 16" as that is what it was going to be, but I moved my screen 6" forward to allow for a 60" LED TV to mount of the wall directly behind where my screen will come down. I don't want the kids rocking the projector 24/7. I did a lot of reading on projector/screen placement and while the 120" is the extreme max the room will support, I think it will work out ok. I know you typically want the bottom of the screen to be no less than 36" from the floor, although mine will be about 34". I think the distance at which I am seating will allow it to work out fine.
Your front speakers are going to be a little over 4 meters away I would guess. While the rule is to subtract 6dB of output for every doubling of distance, that refers more or less to an anechoic, or at the very least, an open field with only the ground reflecting sound. In an actual room, even with absorptive room treatments on the walls, you lose more like 3 or 4dB with every doubling of distance. Let's be on the safe side and say 4dB.
So we have to look at what sort of output and amplifier power you'll need. If we take something like a 90dB at 1Watt/1meter efficient speaker, it'll be producing around 82dB with 1 Watt of power at your primary seat. At the very least, 78dB with 1 Watt. That's not bad at all. You'd only need 2-6 Watts of continuous power to reach the 85dB average for reference level playback and only 200-800 Watts of peak power for the 105dB reference peaks in the speakers. But that's a fairly efficient 90dB/1Watt/1meter speaker. Plenty of speakers are 86dB/1Watt/1meter or lower. Now you'd need a little more than double all of those figures to hit reference.
Let's say you get the $700-ish GiK room kit plus a couple of diffusion panels from them too. You're looking at somewhere around $1200 for that once you include shipping.
Let's get you a pair of highly capable, deep output subs. Some good candidates would be the SVSound PB or PC12-NSD, the HSU VTF-3 MK4 or the Outlaw Audio LFM-1 EX. Once you factor in shipping, they're actually all about the same price at around $750 each - so around $1500 on the subs.
Looking into these options today, while I would like to go with the Rythmik, I am not closed minded.
Denon's 3312 should have ample power provided you go for fairly efficient speakers. What're we lookin' at there? About $850?
Yes close, I can get the Denon 3312 new for $719.00
So we've spent, what? Around $3500 already? Doesn't leave a heck of a lot for speakers out of $5500 does it?
Still, you'd have no trouble affording something like the Emotiva ERD-1 surrounds on clearance for $250 with a really nice front soundstage like the
Ascend Acoustics CMT-340 SE mains and CMT-340 SE center at $794 shipped right now. Could get the dedicated stands for the CMT-340 SE mains as well, which make them look like towers

I'm a big, big fan of Ascend's CMT-340 speakers. That SEAS tweeter they use is WAY above its price point in terms of sound quality. Highly efficient and high output - they'd have no trouble in your room with just the Denon powering them.
Should leave just enough for a decent disc player and remote! Not enough to get a good UPS Battery Backup power protection from APC though. If you want power protection, go APC. No question. The J35B is the best deal, although it's only got 8 outlets and only 6 of those are battery backed up. The J15BLK is the one I really like most. 12 battery protected outlets. And if you wanna go all fancy like, the S15BLK is sweet. Around $500 for the
J15BLK if you search online. Totally worth it, IMO. Especially for a projector (to protect the lamp during a power failure) and for any DVR (let's you keep recording during a black out!) or anything with a hard drive or cooling fan (keep 'em spinning!).
Hope that helps!