You dont need a mixer, no. The DAW and often the interfaces software serve that purpose.
Get firewire, not USB. Peak speeds may be similar but firewire transfer is more stable by far, USB is burst and interuptible by other threads even when the CPU isnt under load. Thats important for audio as it means that USB interfaces either have more dropouts, higher latency or both.
Roos suggestion is a good one. The saffire stuff is about the best out there till you get up to RME, Apogee and the like. Maybe stienberg too, but I know of their interfaces only by reputation.