Well I found this link in Dell supporthttp://support.dell.com/support/edocs/systems/ws690/en/ug/about.htm#wp1056162
I dont see a VRM shown on motherboard in there.
I am thinking that it has a built in VRM?
I also could not find a VRM on ebay or google.
If it needed one it would be for sale wouldnt it?
Internet is mainly used for selling...
Try asking question and searching Dell user forums on their support page.
The guys there live for Dells
Nice tower by the way. I have a precision 450 and I am jealous LOL