See
http://support.asus.com/cpusupport/cpusupport.aspx?SLanguage=en-us. I'd
give a direct link to the page, but the Asus site is not set up that way.
The fastest CPU that Asus recommends for the board is the P4 3.06 with a 533
MHz FSB (front side bus). (Incidentally, it's the only 533 MHz P4 CPU that I
recall supports hyperthreading.)
As another poster remarked, you may have trouble finding one. If you try for
a slower CPU, make sure that it's still a "Northwood" chip (512 kB L2
cache). The ones with 1 MB L2 cache are "Prescott" CPUs, and they are not
supported.
If the best CPU that you can find is a 2.53 GHz one, I doubt that's a
worthwhile upgrade.
If it was my own system, and the gods of overclocking smiled, I'd probably
be running the P4 2.0 GHz CPU at 20X133 MHz (2.66 GHz). That's not a
life-altering improvement, but it would give 87% of the performance of the
fastest CPU available for the machine (the 3.06).
Address scrambled. Replace nkbob with bobkn.