Nothing less than 32mb if you want to be able to game with it. However, with the new stuff that's coming out, you may want to shell out some more and get a 64mb.
If you go GeForce 3-4, do not get MXs, TI's or FX (FX if you have the money). Besides that, RADEON from 7500 and up oughta do nicely.
EDIT: And there's a GeForce 4 out now with 8X anti-aliasing.