he might have a prebuilt with now PCIE 6/8pins connectors.
and no- 4 gigs of VRAM in a low end card is stupid. its a selling gimmick, the 640 doesn't have the pixel rate to fill 4 gigs.
a 7990 in a modded game with 4K textures on a 4K screen? yeah. sure. a 640? It'll burst into flames and explode first.
I'd suggest getting a better PSU if yours isn't good enough to handle the extra draw of a 650ti. a *good* 430 watt (which will handle most basic computers) is around $50 for a modular and $40 for a non-mod.
Brennan will try to tell you a 360 watt will handle any single card set up, but I think he forgets things like hard drives, disk dives (if you HAVE to have one.) and the little draws like fans and RAM.