M-Audio has some PC cards that are pretty decent I have heard. Never tried one yet but I'm looking into this area as well for myself. I was talking with another musician locally about this exact thing and he was looking into just using a USB mixer itself to get his music into the PC. I guess this way you could pre or post effect your guitar sound. The objective I guess really comes down to getting into the computer exactly or as close to as possible the sound you are hearing from your amp or head phones and a mixer might have better control over this because you might have to mic your amp to get the sound you are hearing and then bring it in through the mixer itself or a card like the M-Audio. I would probably use both devices in connection because the M-Audio card is going to have a heavier duty connection that is shielded better than just a sound card will have unless it is one of the more professional ones available. Another problem with USB devices are that you may run into some latency issues with real time recording. RME Hammerfall is another line of equipment that comes to mind, this is a little more expensive but it is professional grade equipment.
Also the Delta line is the same kind of card types as the RME line except they are built a little more on the cheaper side from what I have read about them in articles. both of these product lines support ADAT plugs, take in 24-bit 48kHz sound minimum, and some of them will take 96kHz. A standard sound card does support the minimum as well, usually it's just noisier and cheaper built and the connections are inferior to these professional rigs not to mention that some of these pro rigs have multi tracking hardware on them.

Groove On!!!