Use the following guidelines when choosing an audio adapter:
When you add or replace an audio adapter, don't pay for features you won't use. Don't buy an expensive audio adapter if you'll use it only for playing CDs, casual gaming, VoIP telephony, and so on. Even $30 sound cards include most of the features that more expensive cards provide, and are more than adequate for most purposes.
If you use your sound card for 3D gaming, buy one with hardware acceleration and other features that support what you use the card for. Capable consumer-grade audio adapters like the M-AUDIO Revolution 5.1 and Creative Labs Audigy2series sound cards sell for under $100, and are suitable for anything short of professional audio production.
Stick to name-brand audio adapters. We frequently hear horror stories from readers who have purchased house-brand audio adapters outdated drivers, missing or inadequate documentation, poor (or no) tech support, shoddy construction, incompatibilities with Windows, and on and on. What's particularly ironic is that you may pay more for a house brand audio adapter than for a low-end name-brand card. You can buy decent name-brand audio adapters for $30 from reputable companies. Don't buy anything less.