I play with 4GB, and I can run 1 client with decent success (mid-high graphics settings, no effects, etc). FPS is actually terrible, but I suspect that has more to do with my video card and some persistent heat issues than the RAM or processor. During large fights I often dial down settings to a bare minimum, but things typically stay pretty playable.
Dualboxing (with one client minimized) is possible. Lowering settings is sometimes neccessary here as well.
EDIT - my specs for reference:
Intel i5-520M @ 2.4GHz
4GB RAM in 2x2GB sticks
ATI Mobility Radeon HD 4570