My old PC had a Tualatin 1.33GHz. It had a 5700U @ stock speed. From various tests I made it behaves like a P4 Williamette at 1.6-1.8 GHz, save in memory intensive situations. Coupled with a GeForce5700U I could play Doom and Far Cry at 1024x768 at high detail with 30+fps in most situations.
Pentium IIIs below 800 MHz (especially if coupled with 66-100 MHZ bus) do not saturate a GeForce 2GTS, thus everything above is overkill unless it comes for a very cheap price