As a computer builder here are a few points to clear up that always seem to cause confusion in this type of thread
Very, very Generally:
Sound card what you are mainly paying for on an after market "Audio Interface" is AD/DA conversion. Onboard/Gamer cards do an adequate job of DA conversion but are not really designed for serious AD converion. However it will work for a hobbyist.
Hard Drive Write speed will determine how many tracks can be recorded at the same time, read speed how many played back. An average 7200 RPM drive on a desktop, which is pretty much standard, even with all the junk big box manufacturers dump on them can easilly handle 50 simultaneous inputs and likely a hundred in playback
RAM heavy sample based stuff like MIDI instruments (not synths but sampled instruments being triggered my a MIDI command such as
garitan personal orchestra or EZ Drummer) benefit from more RAM. More of the library can be held in RAM so the Hard drive does not neeed to be paged as often
CPU is for number crunching. VST Plugin effects and synths need to perform calculations on every note that passes through them. More CPU power means you can run larger quantities of these calcualtions at the same time without your computer locking up/freezing/popping/clicking
USB is not limited to a stereo pair input. there are plenty of USB inputs that can give you 4 or even 8 simultaneous inputs. Many USB units are limited to 2 channels due to bandwidth concerns as even USB 2 has a fairly low sustained throuput bandwidth and is more liable to give you pops clicks and dropouts with more than 2 channels. On a well executed USB bus (which is rare/unheard of on a big box brand) there is no reason that USB 2 couldn't handle more than 8 channels
get your computer working and then forget about it, more CPU, Hard Drive power RAM etc will not make your music any better it's just a receptical into which you record. Don't let the nuts and bolts distract you from getting the music right at the source!