It will depend on the capability of your interface. They normally call it 2 x 2 which means 2 tracks simultaneous input and 2 tracks simultaneous output. Other interfaces can go 4x4 or 8x8 or up to 24x8, etc. You will also have the capacity to control the gain since most have built in preamps. There are a lot of interface with 2 xlr inputs with good mic preamps, some even more. Most of these interface have software mixers included so you wont need your outboard mixer anymore as you will control them in your PC. The more advantage here is you have the option to record on 24bit or higher and on 48khz or higher.
PCI is much faster than USB and firewire (but this is a debatable statement - although logically that is how I see it). More processors on your PC would help (duo, quad, i5, i7 etc.). More memory will also help although 2gb is good already. But remember, back in the days, we used to record on windows 97 and on slow PCs but still could get quality recordings out of it so its really up to you how you could maximize what you have. The point here is it is possible to record a decent track with even slow computers and low resolution interface. Convenience is another thing.
I hope this helps.