If you are only going to use one mic, plan to only have a stero mix out from your PC, and things work for you now, I don't see any need for a mixer. Otherwise look into getting a mixer that meets your needs.
I bought an ART pre-amp before I really new that I wanted to get involved with digital recording. Is there anyway I can get around buying a mixer (i.e. use software?)
If I do need to buy a mixer should I just try to sell my pre-amp or would it be worth keeping? I record guitar, bass, drums and vocals.
My set up is as follows:
ART pre-amp
K6-2 PC
DMAN 2044
Sound Forge XP
Cake Walk 9
SM-57