Originally Posted by AgentCash

Opteron is 64 Bit so you'd compare it with Xeon not P4
Sempron is low-end so you'd compare it with Celeron not P4

If I was looking at keeping the server for a while I'd go with Opteron simply for the larger amount of directly addressable RAM, but with server prices as low as they are and CPU speeds as fast as they are, I usually just look for the best bandwidth deal since I usually serve mostly static or cached content. If you're doing intensive database calls go with Dual-Opteron with a shitload of memory. From the benchmarks I've seen Opteron beats out Xeon in most server based tasks, but lags behind in the desktop arena.