Alex is the one to answer such questions, but he's currently out of town, so it might take a couple days to reply. One interesting thing is that MySQL doesn't scale well after four cores (http://anandtech.com/...aspx?i=3162&p=10), so it wouldn't be worthwhile to get more than 4 cores for a dedicated MySQL server. Also, the article mentions that the 5.1.22 release candidates are up to 70% faster than the 5.0.2x release! They don't go into specifics there though.

Adrian