2GB is never going to be enough to run 5 instances of WoW if you ever plan to get out of the starting zone. Each client easily takes up 500MB of memory or more. Besides the 2.5GB of RAM that WoW will be taking up, Windows will also need some for caching and other programs. About 4GB is needed to run 5 instances of WoW and have some memory left for other apps and Windows.
I myself placed 8GB of RAM for 5 clients, so I could disable the swap file and have a performance increase.