you have a host of options. I'd go with an nForce 4 Ultra based board, since this design has been around a while and is well tested.
If you look at Neweggs mobos AMD compatible -> nForce 4 Ultras there are only 15 listed, and almost all are under $100. A friend of mine just put together a machine based on the ASUS A8N-E
and has had solid performance, but any of the name brands there (ECS, ABIT, DFI, MSI,...) I'm sure would be great.
Couple that with 1GB RAM, a 3700+, and a 7800GT and you'll have a great system for under 1K.