Mike, would like for you to know that I have completed and shown stable an original MacPro (1,1) that I upgraded from 2X 5160's (dual core 3Ghz/1333) to 2X 5350's (Engineering samples, quad core Clovertown 2.66/1066).
Via the BSEL mod raising the FSB to 1333 (and hence forcing CPU frequency from 2.66Ghz to 3.3Ghz), and the VID mod to raise the voltage to 1.425, the system was stable except for heat issues.
I designed and home-built a geo thermal ground loop with a 40ft. section of 1/2" nominal copper pipe exchange. Cooling blocks for the North/Southbridge chips as well as both CPU's were Koolance products modified for the MacPro's unique case. The GPU block was an unmodified EK 5770.
This actual "experiment" was to find a way to cool my office as well as to demonstrate water cooling in the Mac Pro, and further modifications will be made in the future in attempt to capture more heat (particularly that from the PSU and RAM banks) by the water system.
I hope to have a full write up available in a week or so (I'm so behind in everything now!), but for now I can only offer a proof of concept on youtube.