well, Dual P3's if you aren't into overclocking..
There are many people who have gotten their P4's to over 1800mhz.
At that speed, it would be the fastest affordable processor on the market.
And remember, a dual setup will only speed up certain applications, or speed up multiple cpu hogging applications that are running simultaneously.
otherwise, for all intents and purposes, a single fast processor will yield much better performance.