I'm going to recommend you something quite different to what they have said above, as you are wanting to buy speakers for gaming. Its called the Philips amBX Premium kit, which gives you a 2.1 set of speakers, with a wall washer, a wrist rumbler and 2 fans. The speakers and the washer both have ambient lighting on them which change colour (there are around 2 million different colours) whilst you are playing games, the fans will blow into you when running in a game or flying etc. and the wrists rumbler will activate and make you feel like you are firing a bullet from a gun. Not to mention, the speakers give great surround sound and bass. I've played Call of Duty 4 with and without them and believe me, it changes so much and really makes you feel like you're really there, firing a colt or a sniper rifle and hiding in the jungle. They have recently dropped in price, and for all of the above it only costs around £170 from play.com.
If you want to take your games to the next level, please give them a go. You can find out more details at www.ambx.com.