Top One:

http://www.xlr8yourmac.com/ The Source for Mac Performance News and Reviews, very extensive site with pro reviews, advice and news.

Links selection:

http://www.macspeedzone.com/ very extensive site, very usable, a true resource for performance in the Mac world

http://www.mac-upgrade.com/ dedicated site where you'll find an impressive news section, reviews and opinions on improving your macintosh. 

http://www.powerbookcentral.com/features/cpu.shtml powerbook specific upgrade site, very useful information and expert advise