I'm currently running an i7-950 with 12GB RAM - with 5 WoW sessions open, I see about 8-9GB being used. The benefit of having 12GB shows when opening other apps (several IE windows, media player, excel, email, etc) - there's no reduction in the RAM available to WoW, so less impact on performance.

If you want to see how much of a limitation your video card is - download an app called GPU-Z (http://www.techpowerup.com/gpuz), run it, and see what % of your GPU is being utilized when you multibox WoW. if it's near or at 100%, then you should probably look at upgrading your video card as well.

GPU-Z and it's sister app, CPU-Z are great for testing the utilization of GPU's and CPU's respectively, if you're looking for bottlenecks in your system (they provide good info if you're overclocking as well).

Cheers,
S.