The 2GB version is worthless (literally) unless you want to play games on multiple monitors, or have a monitor of extremely high resolution.
1GB is plenty even for 1920x1200 resolutions. More VRAM is basically pointless.
Now, there is a performance difference between the GTX 460 and GTX 470. Which one is best for you depends on what games you play, and what resolution your monitor is. A GTX 470 will plays most games at 60+FPS at 1920x1080 (exception being Metro 2033 with DX11 turned on).
If you don't demand absolute highest performance & highest graphic settings, or use a monitor with a lower resolution, the GTX 460 1GB is plenty.
We'll I go with the 470 to future proof the system.
I'm getting the from ibuypower and the 460 1GB is only 10 dollars cheaper than the 2GB version.