Well, I just replaced my stock 2.8 GHz CPU in my 5,1 with a faster one, and thought I'd briefly share a mistake I made in case it helps anyone else. When removing the old thermal compound from the heat sync, I used a coffee filter because the Arctic Silver instructions say it is acceptable to use and is lint free. However, it seems to have scratched up the copper on my Mac Pro's heat sync quite a bit. I didn't feel like trying to figure out how to "lap" the heat sync to smooth it back out, so I proceeded with my installation and it seems to be working -- but I'm not happy about the scratches. Perhaps a micro fiber cloth would have been better. Edit: I guess I don't know for sure that the coffee filter caused it, but I did wipe in a circular pattern just like that.