Depends on what you want to do with it. If you're only running a few applications at a time, 2GB is plenty. I run Win7 on a 2.0GHz Core2 Duo with 2GB of RAM and it does a decent job.
A 3.4GHz P4 oughta do a decent job also, as long as it's got plenty of cache. Won't be a speed demon, but IMO it's worth trying.
That's about it. I have to wait to replace my video card before I can try it (I'm running a very old Quadro 64 MB card since my GeForce went belly up).
My processor has 512KB L2 Cache. What do you think?