Socket 939 is being phased out, so while it's still a viable option, do know that CPU's faster than the FX-60 (which is no slouch) won't be an option. AM2 is not compatible with Socket 939 (Athlon64, Opteron 1xx) or Socket 940 (Opteron 2xx and 8xx). The only thing Socket AM2 does is make the jump from DDR to DDR2, due to AMD changing the memory controller on the Athlon64 design to DDR2. Both Socket 939 and AM2 support the X2 dual-core CPU's.
Your best bet is going to be something based on an nForce 500 series chipset. The 550 is the basic unit, followed by the 570, 570SLI and the top-end 590SLI. There's also a few motherboards based on the older nForce 4 chipset, which is another option. The nForce 5 has more bells and whistles than the nForce 4 series, but outside of that, you're not going to notice a whole lot.
Any board with at least 1 PCIe x16 slot will fulfill your needs.
Foxconn, Asus or Gigabyte are easy to find and should be some of the easiest boards to drop in and go.
While I know you're looking specifically at AMD, Intel has a nice CPU in the Core 2 Duo. Their E6400 is comparable to the Athlon64 FX-60 and is cheaper (unless AMD does a major price slash). Look at some reviews around the web and you'll see what I mean.
