stanley85 I know it seems counter intuitive, but trust us on this one (or don't, and track down some of the VGA charts.)
The size of video memory means jack squat compared to the difference between DDR2 and DDR3. I remember back in the day my brother's 64MB TI4200 putting 512MB FX5200's to shame. Size these days is mostly marketing, unless you are cranking really really high resolutions (like, 1650X1050 or higher) and even then, I'd still go with the 256 version, because the faster memory will more than make up for the size.
Think of it this way, that extra space will give a tiny boost (1-2% maybe) to your card SOME OF THE TIME, but extra speed will give an enormous boost (probably 30-50%) ALL OF THE TIME.