I plan to rebuild my render farm (2 P4 3gHz w/ 2gb RAM each and 2 AMD 2gHz 1gb and 512 mb RAM respectively). I have found that rendering time is a fraction of what it was before. Performance manager only shows 24MB of RAM usage but 100% of CPU cycles. So I will get the most powerful processors I can afford but only 512MB each.

Either that or puchase a lot of 40 866mHz machines off Ebay (they're like $45 each).