Yea that setup will work, Just buy 2 keyclones, install 1 on each machine and setup(stickies are up to show you how). Slave machine quads are good for 4 clients just dont run SLI mode on it it will kill your FPS. OS- xp and vista32 4gb ram max, any more ram and you will need a 64bit OS(Vista64)or its just a waste of ram.How many monitors? 2 will work fine, just use keyclone to setup your screens on the slave machine(stickies there to show u how).

Me personly on the slave machine running the 4 clients I would do 1 install and make 4 other directories for the 4 accounts and symlink em to the main install directory.