|The good ol' Socket 478.|
The fastest processor is the 3.4 GHz Pentium 4 Extreme edition.
After that is the 3.4 GHz Pentium 4...
...then the 3.2 GHz Celeron D...
...and the highest-clocked lowest-end processor for the 478 is the 2.8 GHz Celeron.
If you go to eBay, you might be lucky and get a P4 Extreme Edition for cheap.
Hope this helps,
"If at first you don't succeed, skydiving may not be for you."
-Our tour guide at Fenway Park in Boston, MA.