One thing I will chime in with is that I was never a fan of the Sun/AMD x4200 servers. Oracle On-Demand was using these and I found on random servers that they held a load of 3 for no reason. The best that I could find on the web (as I didn't have root access) was that it had to do something with Redhat 4 and Sun's Virtual drivers in the Bios. I would recommend anything but these servers, besides the new Intel Xeons blow away anything that AMD has.
Intel Xeon and AMD Opteron Battle Head to Head | Tom's Hardware