Hrm... well, at this point, I'd have to say, that either would work just fine. As to performance, both will perform just fine. The 1.8A will overclock real nice, but running a server requires top-notch stability, so that shouldn't be a concern. I would go for the XP, only because I feel that it isn't until 2.2GHz that the P4 is able to consistantly outdo an AXP (at that point, the MHz difference is enough to give the AXP problems). But, you really wouldn't notice those small differences, so it's a matter of taste, ability, and needs. Either would work very well.
Actually, a gaming LAN might benefit from a dualie setup (correct me if I'm wrong, people) with WinXP Pro or other multi-CPU supporting OS, in which case, it's the Athlon MP all the way, since I know of no good P4 dualie solutions that don't require purchasing the expensive Xeon processor. Also, I think that RDRAM only comes in up to 512MB sticks, while DDR comes as high as 1024MB, so you can max out a board if you need to (and you might want to, if it's going to be a server)
My only stipulation was echoed above: if you are new to system building and unwilling to go slowly and carefully, just get a P4 and save the forum another "AMD sucks!" post.
-SammyBoy