Congratulations on the nice TV. The little Onkyo 600 series are nice little receivers and I'm sure you will like it a lot. Just be sure that you stick with 8ohm speakers when you're out speaker shopping.
A good 5.1 speaker system, for $800 is a tall order. Just a subwoofer big enough to properly fill almost 4000cuft will set you back at least $500 plus shipping. If it were me I'd start by building the best 2.1 or 3.1 system I can afford and then go back and add the other pieces and maybe a second subwoofer when you have more money. Anyway here are two subs to start your search with. Hsu research is having a sale on their plain black
VTF-2 MK3 (rated for up to 4000cuft) but that kills your budget. You could probably get by with a smaller
Elemental Designs A2-300 but don't expect truly room shaking bass until you add a second one later on down the road.
The problem with recommending speakers is that they are very subjective. We all have different hearing and taste. Some like a somewhat bright speaker. Others like prefer a warmer sound. It just depends on your taste. (
audio terms) All I can do is give you ideas about what to go listen to and then you need to get out and listen for yourself. Just keep in mind the tricks stores use. When comparing two speakers the loudest will sound better so when doing an A-B comparison make sure they are level matched. Make sure that the receivers are set flat (no tone controls in use) and no processing is being used. And be aware that the room acoustics and thus the sound of the speakers may be totally different once you get them home.
A popular recommendation for an excellent bang for the buck speakers that work really well in a large room is
Infinity's Primus P362 tower speakers. You find them between $150 and $200 each depending a sales going on at the time. I'm told that they do benefit from a simple and cheap DYI mod but haven't heard the modded version. Anyway these should be really easy to drive loud and clean with that little Onkyo. Later when funds allow you could go back for the timbre matched
PC350 center. And later down the road add a pair of P162 bookshelf speakers ($200/pr) as surrounds.
I'd also listen to some
Boston Acoustic CS26 bookshelf speakers. You can find these online for as low as $80ea in black. I use them in my nearly 3000cuft master bedroom and drive them with the older Onkyo TX-SR606 and they sound pretty good for the price when backed by a properly sized subwoofer. Combine them with the timbre matched CS225 for about $150 and you have your 3 fronts for around $300. However these are rear ported so they do need some room behind and around them. Later you can add two more CS26s as surrounds.
I'd also give some Klipsch RB10 bookshelves ($250pr) a listen but they aren't for everyone. Klipsch tend to be easy to drive and very detailed but sound a bit bright for my taste. These are also rear ported like the Bostons and will need a little breathing room. The matched center is the RC-10 center (~$200).
You can watch Newegg.com for their fairly regular sales on Polk speakers. It seems like they are always having sales on one or another bookshelf, tower, or center. You'd have to piece it together but you could save some money.
Finally, members often recommend Behringer B2030P studio monitors ($131/pr shipped) as left, right, center, and surrounds. I love the ones in my office but I think that your room might be a tad too big for them. They are also a bit too industrial looking for most wives.
What I would avoid like a plague are small satellite speakers. You want something that will allow you to cross over to the subwoofer at 80hz or lower. That's the point below which it becomes harder to tell where the deep bass is coming from and you avoid the sonic equivalent of watching a ping pong game as the sound moves from the speakers to the subwoofer and back.
Oh and you'll save a ton buying cables and such from monoprice.com.
Good luck in your hunt.
