Q6600, 3gb RAM, Geforce 8600 512mb, Windows XP = at least 4 clients at 1024x768 with settings auto optimized for performance

I could probably run 6-8 clients but never tried.

My spare PC (Athlon XP 2600, 2GB RAM, 256 mb graphics) can barely run a single instance of the client since they removed the 'classic' version.