I don't think the lag we suffer in Shatt has anything to do with the performance of our video cards.

Rather, as Shatt is so densely populated, there's a hit because of:

Increased network traffic (there's more going on that our clients need to be told about), and
Increased disk/bus activity (more people wearing a greater variety of clothes means more models and textures that need to be loaded off disk and into our video cards).

If your video card and memory weren't the bottleneck before the upgrade, upgrading them won't help.

Similarly, Vista 64 won't help you as WoW is a 32 bit app and Vista will run it in 32 bit mode (see http://en.wikipedia.org/wiki/WOW64).