I am running Vista 64bit with 8GB RAM and have no problems with five epic flyers.
Cheapo Q6700 quadcore, 1x 4850 graphics.

I'd recommend to stay away from 64bit XP, the versions that exist were never meant for gaming.
You do need 64 bit to use more than 3.xGB ram though and five times Northrend uses between 6 and 7 GB RAM on my system.