Get nforce 2 chipset (especially since you already have ddr 400 ram). Don't know any other chipset that provides dual channel memory bandwidth for Athlon Xp. If you don't get dual channel ddr memory, then you're not going to fully utilize a barton chip (or any athlon xp for that matter). A great motherboard that comes to mind is Asus a7n8x series. You can overclock any barton core to XP-3200 speed (full 400mhz FSB with 1:1 ram speed).

True story:
Installed athlon xp 2200 cpu and cpu die didn't make contact with heatsink (think heatsink was backwards). Powered PC and wouldn't boot. Removed cpu, looked underneath, and cpu had black burn mark. Only took a second for it to happen. Returned cpu, got a full replacement free, bought a copper shim to go with, installed cpu with shim and heatsink; no problems.

Actually dual channel is of very limited benefit unless you get an nForce IGP board. The Athlon CPU bus is 64-bit DDR just like a single channel of DDR RAM. The IGP though really benefits from that extra bandwidth for 3D performance.

More important for the CPU is memory latency. NVIDIA's nforce2 memory controllers are the best produced for Athlon. Their AGP performance is too. VIA AGP tends to cause strange 2D/GUI performance problems for example.