For AMD CPUs my favorite boards use the nForce chipset. Since nForce 2 just came out, they are hard to get. I really liked (and use myself) the Asus A7N266-C. The nForce 2 equivilant is supposedly called the A7N8X. Unfortunately, I haven't had any time to try one out yet.
On P4s, I'd go DDR Sdram. Rambus is slightly faster, but significantly more expensive. The i850E chipset is also not as feature rich as the newer 845 series. I'd look for a i845PE based board for the best chance of future CPU support.
