The Athlon XP 1800+ (1.53GHz) is currently at the price/performance sweet spot, costing ~$100 for an OEM (add $5-10 for retail), but you can get a 2100+ (1.73GHz) for ~$200 for either OEM or retail. The actual performance increase from a 2100+ over a 1800+ is ~6-12%. As for motherboards, you should consider the following boards:
Asus A7V333 RAID, USB 2.0, Firewire, CPU Overheat Protection, C-Media 8738 6-channel sound (many claim they like it as well or better than a SB Live!) ~$140
MSI KT3 Ultra-ARU RAID, USB 2.0, C-Media 8738 6-channel onboard sound ~$115
Gigabyte GA-7VRXP RAID, USB 2.0, Dual BIOS, PCI128 onboard ("PCI onboard" uses a PCI master) sound, some people have complained of BIOS problems with it, Rev. 2 of this board due out mid/late June will include CPU Overheat Protection and hopefully fix any BIOS problems ~$110
You can get PC2100 RAM cheat at both newegg.com and googlegear.com. Consider using good PC2100 instead of PC2700 RAM unless you plan on overclocking, as the performance increase is nominal at best.