Read this
modding Athlon XP's to work in SMP mode article over at HardwareZone.com.
As for the motherboard, you'll want to read reviews and such about boards based on the 760 MPX chipset from AMD.
The Antec 380W TruePower, that's a good power supply and it will most likely suit your purposes well. Unless you plan on running like 3 CDROMS and 6 HD's and 4 neon lights 380 watts is plenty even for a dual system. I am running a Dual P3 with 3 CDROMs and 3 HDD's off a 300W power supply and it suffices just fine.
The AMD 2000+ (1.6 GHz) is a very good chip and has a lot of overclocking potential if you're interested in that too. A lot of people will recommend those as the best value. I just bought myself a new 2400+ (2.0 GHz) yesterday, but I'm waiting for Mr. FedEx to deliver it this week. Your choice for processor depends on how much you're willing to spend, if you are interested in overclocking it, and if you willing to mod the chips in SMP chips. If you decide you aren't interested in taking the chance in modding the chips, spend a little extra and buy the MP's and then you have nothing to worry about.