I've been running a Dual Xeon 400 for about 5 years now. Its been pretty bulletproof. I ran for 2 years with LINUX on it and then the last 3 with Win2K professional.
Not many games are built to take advantage of dual processors using MPS (multi processor support), but if you're multi-tasking, they do help.
You will need LINUX, UNIX, Win2k or WinXP-Pro to actually utilize the second processor. Win98, WinME, Win95 and WinXP-Home do not have MP support.
Also, you want to go with a good motherboard. I recommend the SuperMicro products for dual Intel processors. They've been doing it for a while and they have quality products as well as quallity support. However.... You'll most likely pay twice as much for their gear.
AKDejaVu