I run 6 instances on:

Windows 7 64bit
i7 920 @ 3Ghz, no HT
12G mem
Ati 4870 1G
2x OCZ vertex ssd in raid 0

I have 5 boxes at semi minimal settings and no multicore from 1 folder, and one at max graphs options with multicore from a seperate folder.

With my old graphics card (8800GT 512Mb) i had trouble with textureloading, after the upgrading to a new gfx card i now run smooth, bar the odd loading time when i run very fast through open space with abnormaly many textures loading.

The reuse textures option in the graphs setting help to minimize textureloading.

I detect no memory leakage whatsoever, even when leaving the team logged in fer more then a day.

I should say your system could run 3 boxes at minimum settings.