I am not really sure that I needed another upgrade, but I went for it! 

Added this round: 

Another Side mount CPU rack on the right hand side. This one is holding a new high end PC that matches the one on the left.  I left the cabling long enough on this one to reach to a seconday desk for 2player Head to head FPS style games. 

A 24" monitor in the center. The 20" I was using was getting a bit cramped and lets face it, I am not getting any younger and my eyesight none the better. I suspect I am getting very close to the weight capacity of my desk.

Tucked behind the monitors I added the Phillips amBX lighting system. Cause mood lighting is cool.

In the center position of CPU's I've repurposed one of the old PC's to a Windows Home Server, added tons of Storage capacity, moved all my media files and am running automated backups and restore services for the other computers.

4 out of the 6 Gaming PC's are now running Windows Vista 64bit, thanks to a subscription to TechNET plus.