Your Ultimate Linux Box will probably have a lot of sounds going on; you might be tailing important logs through Festival while you're listening to some Ogg Vorbis files, while your window manager is playing little “foosh” sound effects when windows open and close. Then you take a break to play a game; you need a sound card that allows multiple processes to write to /dev/dsp at the same time. Get the SoundBlaster Live!, which allows 32 simultaneous opens and is well supported by both ALSA and a driver distributed with the kernel. I installed ALSA, which has more featureful mixing capabilities.
You can get several versions of the SoundBlaster Live! card—one includes a little front-mounted panel with headphone connector, volume and other doodads, which might be handy for basic audio needs. If you have multiple systems with audio that you want to hear, or want to do some audio recording or streaming, make room in your budget and your workspace for a compact mixer, such as the Mackie 1202-VLZ Pro. It's under 400 bucks in the studio section of your local music store and lets you hear all the action on multiple computers, as well as mix the audio you want to record or stream.
(Since you're buying this Ultimate Linux Box to support your web site, it's worth getting the mixer and learning to use it just to check out how well Mackie uses the Web to educate customers. Mackie's web crew must be regular passengers on the cluetrain.)