I'm just getting into this myself, i have a P4 3.2, D865 Intel Mobo, 1gb 400mhz ram, two 80 Sata II HDDs, and a Radeon 9000 series (not sure which one off the top of my head)
I don't need a ton if in/outs at the moment so i went with the E-MU 0404 sound card, you can get them for $75-$99 and they're supposed to be great cards as long as you have a decent PC to run em.
The Audiophile 2496 is pretty much the same thing as the 0404, except it has lil' RCA outs as opposed to the 1/4 TS outs of the 0404 and maybe better drivers depending on your system.
Both have unbalanced outs, so be careful if radio interference is a problem where you live. If it is, you might just want to save up longer and invest in a card/audio interface with balanced outs. I think the E-MU 1212 has balanced i/o
You should invest in a pair of monitors(speakers) too... i got the KRK Rokit RP5 monitors... $135 a piece with full warranty.
Are you going to be using soft synth stuff mostly? I plan on using Reason/Cubase/FL6, and a bunch of Native Instrument VSTs. You can just use your computer keyboard but most people prefer a midi controller and i already have my eye on one... haven't bought it yet though.
If you are going to be recording live instruments at all, you will need to invest in an external mixer with decent mic preamps...
Ok so i'm just in the purchase/research/experimentation phase and all of what i've said thus far might be bogus, but this is a good site to go check out...
http://www.tweakheadz.com/guide.htm