Acoustically Challenged Gaming Room

Budget?

It sounds like you really do want to blow the doors off. However, a 5.1 Cerwin Vega! setup is going to be close to your entire budget if you include the VE 8" towers, VE 5M bookshelves for surrounds, and the VE-5C center. This goes for about $630 on Amazon. You would still need a subwoofer and of course a decent receiver. Perhaps you could build slowly by starting with 3.1 and expanding later. For the subwoofer, I would recommend the HSU VTF-2 MK3 or offerings from Rythmik, SVS, or Outlaw. Add the Yamaha 667 for the receiver. The 667 has pre-outs so you can add another amp if you think you need more power (which you might in that cavern).

Another option would be to go with a 5.1 setup from Ascend Acoustics. I think the CMT-340SE towers could fill that space. You could start with a pair of towers and a subwoofer and expand from there. Ascend will still give you the package price even if you don't buy it all at once. For this configuration, a Harman Kardon 2600 receiver might be a good fit.

Check accessories4less for their factory reconditioned Onkyo receivers. I've helped relatives buy from them a couple of times. You can get a good deal. Once you select your receiver, that'll give you the baseline of what you have to spend on speakers.

And then as then as postrokfan explained, you might be best to start off with a 3.1 system. I would suspect that if your boys are like my sons and his friends, they will be more impressed by loud, deep bass effects than SQ out of the main speakers. You could probably could even get by with a good 2.1 system with a solid sub. Or at least a 4.1 and skip the center channel at first. I'd suggest considering a 12" sub to get that rumble in the garage. BIC is well-regarded for 12" subs in the $200-$300 price range for home theater (there's an owner thread you could check out here). Or you could go a little higher in price with the brands postrokfan suggested. Check out Emotiva's Ultra 12 which is $429 with free shipping.

Be sure to get speakers, though, that have a higher RMS wattage than the receiver. Boys are bound to run the system up pretty loud when you aren't around and test it's limits. For instance, HSU, who is well known for their subs, makes a L/R channel and center channel speaker that can handle fairly high wattage and might be able to survive boy abuse Or even cheaper, you might check the Infinity Primus series of bookshelves. They tend to get good reviews, and are fairly cheap on Amazon and Crutchfield.