My advice is to get the Soundblaster Live X-Gamer (4.1, not 5.1) from staples for $80, use one of the $25 off 75 coupons, and bring it in at $55 shipped free.
I think 5.1 for computers is silly hype. I think 4.1 actually may sound better. The center channel speaker just muddles the sound seperation of the two front speakers. I mean they are probably just on each side of your monitor anyway. And the same distance from you as the center would be.
I have a full blow high end B&W 801 speakers with Velodyne subwoofer surrond sound system for watching DVD and other movies. There a center channel makes a big difference. I mean the front speakers are consideraly deeper than the big TV, and some distance appart. Dialong sounds floating and weird without the center channel.
But none of that applies with computer surrond sound. Sure, the subwoofer and the two rear surronds make a big difference in games or dvd watching. But loose the center. (Dialog is carried by the two fronts, and seems in between them.)
I'm getting the Cambridge FPS 2000 4.1 speaker system, after hearing it at a friends. Nice pair up with the Live 4.1 sound card.
My .02 cents after a lot of reaseach and listening.