First I'd look for one that has the right connectors.
1) PCIE or PCI depending what you have on your motherboard
2) Headphone amp, 7.1 analog out, hdmi, or 2 channel depending on what type of inputs your speakers can take (a lot of people overlook this.)
After that, look for a good SNr (signal to noise ratio). Anything over 100 db is pretty good.
If you want to be able to tweak the sound, read up on that. Equalizers, Crystalizers, and so forth.
If you don't know what you want, go for something cheap like the xonar dg.