The Athlon XP is almost the same as a normal socket-A athlon. AMD made some changes to the Athlon that nets some nice preformance gains.
For example, an Athlon XP 1.2 is almost equivilant to a normal Socket-A athlon 1400.
When coupled with DDR RAM, the XP preforms extremely well (The preformance gain from SDR to DDR is much greater than with the normal Athlon).
AMD is using a rating speed for these processors, so the XP 1800 is really a 1.53 ghz chip. AMD feels they need to rate their processors because people will look at the MHz number and then get a P4 because it is higher.
Almost any Socket-A motherboard will work with the XP. The newer ones will work for sure. Older ones will need a bios update.
Hope this helps.
Oh, and the XP should be showing up any time now.