CompUSA, at least their website, has a good selection of products. I never realized how many A/V products that they carried.
Something to consider with using the left/center/right set up with no rears that you're talking about for FPS (based on your thread about a monitor) is that you may not hear the people coming up behind you. For all I know, there are sound modes that just use those three speakers, but I didn't find that on my system a couple of weeks ago when I was playing COD4. I have the same set up (left/center/right with no rears), and I set it to stereo so that I could hear everything. Just a thought.
That said, you could go with a 5.1 or 7.1 HTIB and not use the rear speakers, or a 5.1 or 7.1 receiver and just not hook up any rear speakers. I mention that in case you were thinking that you couldn't use a 7.1 receiver like the Onkyo 506 (not to be patronizing...it's come up before).
You could also just go with a nice PC speaker system. How close will you be to these speakers? Some of the higher end PC systems are quite nice. I'm happy with my $80 Creative system that I bought back in 2004.
EDIT: I see that Sheep already mentioned that last option.