No... I would not recommend our fellow team member replace the mobo if he already has shown that he is unwilling to upgrade the PSU. I only say this because he will likely also have problems on reboot with the OS on the machine.
I suggest keeping this very simple for him and stick with the Intel E8400 Socket 775 processor. The Intel E8XXX line has a proven track record with SETI and should give him the boost he is looking for with minimal effort.
In regards to Ebay.com, the auctions for the Intel Quad Core 2 Duo processors are smoking hot right now. I have been trying to win an auction for a E9400 & E6600. I just lost the bid of $200 in the last 30 seconds to a automated post of $275. An auction for a rare E8400 E0 stepping is going for over $84 right now (well, that's my reserve price actual is $77 right now).