Both the A64 and Opterons seem to overclock well. I'd go with the Opteron. Reading around these forums, it seems the Opterons overclock better than the A64 so far.
Are you interested in Dual Core? If so, I'd suggest and Opteron 165 or 170 coupled with DFI-NF4 Ultra (you can easily mod this for SLI) and 7800GT (best bang for buck for high end vid card). Save the rest for good RAM and PSU (IMPORTANT!@!@ my PSU is the highest temp in my system when I overclock).
Other options:
CPU: Athlon X2 3800+ (these seem great overclockers also), Opteron 140 + series (single core)
MB: MSI Neo (no confusing configs like DFI), ASUS (it's ASUS)
RAM: Depending on the sizes, 1GB Modules Samsung UCCC or Micron-5BD. Either get G.Skill, Crucial, OCZ, or Mushkin.
512MB Modules, Samsung TCCD or Winbond Bh-5/6/UTT
Good luck.