PCI or ISA?

Soundcards are available in two formats - ISA and PCI. My advice is to purchase on of the more modern PCI cards as they are easier to install.

Wavetable?

Yes! Do purchase a card with built-in wavetable support - most cards these days do, but the venerable Soundblaster 16 does not.

Surround sound?

This is the latest craze - surround sound for your computer. If you're into games, imagine hearing your enemy approaching you from behind - certainly adds to the atmosphere. The best implementations use 4 speakers - two in front, two behind - to give a real 3D sound-field. Some cards try to perform smart tricks with just 2 speakers, but the effect isn't especially good.

Professionel sound quality?

If you're serious about music on your PC you need to buy a card with digital input and output. One such card is the full version of the Soundblaster Live. The card should also have some RAM onboard allowing you to upload so-called sound-fonts. My guess is that if you need these facilities you already know a lot more about it than I do! :-)