it's a time of flux where things haven't settled yet with the new tech. i.e. there is no great, trouble-free AM2 or Core 2 board at the moment (although after some further BIOS tweaking some of the current boards may turn out to be gems).
Or as wusy suggests you can go with known good stuff from the last platforms such as the EPoX, Abit KN8 Ultra (there's a new passively cooled version), DFI Ultra D etc.
X2 3800+ is a good choice & with 939 if you have PC3200 you get to reuse your RAM.
OK, this is the bugger I want... AMD Athlon 64 X2 3800+ Manchester 2.0GHz 2 x 512KB L2 Cache Socket 939 Dual Core Processor
Sure. That's state-of-the-art 2005 equipment... it should be nicely inexpensive!
Actually the AM2 CPUs cost a bit less than 939 (as they are mostly in stock).
So make sure you also get only DDR support, and AGP 8x is a must for a proper older build...