Next look for external controls. Volume control is pretty standard but equalization and panning controls are very nice to have. Some speaker sets even have mute buttons and preset EQ buttons for extra convenience. Also, look for 3D sound capability. Many of today's games use 3D surround for extra realism and you want a sound setup that can tap into this.
Some speakers feature some type of extra bass boost system.
Whether it's MegaBass, HyperBass, SuperBass, NastyBass, or ThirdBass, it's a good feature. Finally, look for as many line in and outs as you can find. A headphone out is a must. However many speakers also have a mic line throughput on the front, a very convenient touch, and some have a second line in on the back for an additional sound source, say for your Discman when your CD-ROM is already full. Altec Lansing has a nice line of computer speakers in the PowerCube series.