The T5500 was available with 2 graphics cards of up to 150 Watts each, so for instance a Quadro 4800 is rated at 150W and has only one 6-pin (75W) PCI-e connector. Notionally, it gets the other 75 Watts from the slot. Since my box only has one hard drive, I have a few leftover 15-pin SATA power connectors - so I use a dual SATA power to 8-pin PCI-e to get additional power to GPUs as needed. I've had an OC'd 7950 and OC'd 7790 installed together - together they exceeded 300 Watts but I never had any issues.

You're right - I don't believe you can OC on a Dell workstation/server mobo. What you give up in tweakability you gain in stability. To me, having the most "up" time is most important for a crunching rig like this.

Good luck!