If you're not into 3D games and want to save the money, get the V3. The two cards should have essentially the same 2D performance. Some people claim that the nVidia chips don't deliver on the 2D image quality, but both cards have a 350 MHz RAMDAC that should deliver the same refresh rates for whatever resolution you're running.
If you're partially into 3D games and want to save the money, get the V3. The 3D performance is not as good as the GFMX2, but the V3 is not a slouch by any means...until T&L comes into play.
If the games you're into run better with Glide (e.g. Unreal Tournament), get the V3.
Otherwise, I think the GFMX2 would be the way to go.