Few Q's about Audio interfaces.

Hi there, id like to buy an audio interface but im a bit confused about a few things.

I wanna record guitar, midi and mic into cubase - 1 at a time.

I have an SM58
Epiphone LP
Casio CTK-650
Xp sp3
3g HT
!g Ram
USB 2.0

Im gonna be building up to 16-20 tracks (VSTi's/Audio+fx)


Do USB.2.0/FireWire audio interfaces help DAWS with processing power? Or or is it only pci's that do that?

I wanna get usb because they come with preamps, will these units be ok for what i want?

USB 2.0
Emu 0202
Emu Tracker Pre USB 2.0
Tascam US122L

I just dont wanna have to buy a separate expensive preamp, if i can help it.

Thanks guys!