Mostly any socket478 CPU should work, but it depends. I think some of the earlier GX270s will only take up to 2.8GHz (it will take faster, but it will only run them at 2.8GHz in that case). Otherwise, a 3.2GHz or 3.4GHz Pentium 4 should work fine, and theoretically, but unconfirmed, a Pentium 4 Extreme Edition should work.

Most of these faster CPUs, especially if it's a Prescott, will probably need the two copper piped heatsink, becuase the standard one won't be enough to keep it cool.