GTX560 Ti DCII TOP/2DI/ 1GB or 2GB

Depends, if your playing at 1080p just get the 1gb version, no need for 2gb vram.

^^ he is correct,

but if you ever want nvidia surround using 2 560ti's in sli, 2gb will be handy.
if you dont want surround 1gb is enough  

Depending on what games you play - BF3 will use plenty of vram - newer games will likely be vram hogs as well. Your choice. If you plan on upping your resolution vram use will also increase. Are you set on nvidia - HD7850 is a nice card and the new GTX660 will be more efficient as well - who knows when will it be released?
-Bruce