Concerning C2D Quad-core replacements:
After looking at the CPU specs in detail for the E7600 and E8600 C2D parts and comparing them to the various available C2Q parts, it seems to require one of the 45nm QxxxxS parts to even have a chance of working, with the Q9505S or Q9550S being the most likely of the higher-end ones. The power regulator specs/etc. are different for the non-S parts.
I'd be happy to try it, pretty sure the worst thing that will happen is that it won't boot due to lack of C2E (Stop-Grant) support or confusion about having 4 cores instead of 2. It could also come up but refuse to recognize the other 2 cores.