Yup, server all the way. The easiest to get is:
An AMD 4p, quad socket, g34 setup running 6174 opterons
After that, running the lowest timed ram, 7-7-7-18, of any compatible ram, enouh to fill all slots, so 8Gigs
The rest of the parts are whatever else you want.
Now the ultimate would be a 4p intel 10 core/20 thread for a total of 80 threads, but thats easily $60K
The AMD build will do 5-600K ppd at about 700W
As of right now, Ubuntu 10.10 is the ticket. but that could easily switch to windows in the next core development.