I quad box on a single machine and I have no trouble with 2GB of RAM. Just a little choppy in Ironforge and Stormwind. I do use separate WoW directories though. I'm not sure if that helps or not, but I have plenty of space, so I never tried just using a single directory.

Nvidia 8800GTX and Intel Core 2 Quad Q6600, btw .... but only 2 cores seem to get used, even with 4 wows running.

I use Keyclone's maximizer function to have the windows in the 4 quadrants of my 24" widescreen. So each window is 960x600. I have all spell effects down/off, except draw distance.