If your psu is up to it then the X1950pro is the best card.
It is a lot faster than either of the others.
The 7600GT is next and in last place the 7600GS.
The onboard RAM is used to store texture information and frame cache (as one frame is displayed the next is rendered into cache ready to be displayed).Very few games at the moment need more than 256MB of onboard memory to do that.
Notable games that do are F.E.A.R.,S.T.A.L.K.E.R. and the DOOM3 engined games.
If you go for the 7600 series of cards then the GT with its higher clock speeds will give the best results in terms of frames per second.