I can't help you on the P4, but my friend has a Celeron D 526 (I think) running at around 2.8-3.0 with DDR ram and it's averaging somewheres around 250-300 RAC.
For some other numbers I can give you, here are other rigs in my farm:
QX9650 at 3700 - 5200-5300 RAC
Q6600 at 3510 - 4000-4100 RAC (not sure if it's topped out yet)
E6750 at 3600 - 2050-2200 RAC (still ramping up I think and had some power issues lately)
E6300 at 3465 - 2150-2200 RAC
E6600 at 3520 - 2150-2200 RAC
Opty 165 at 2650 - 1150-1200 RAC
P3 1.0 laptop - ~130 RAC
X2-4400+ at stock 2200 - 900-950 RAC
Dothan 2.0 GHz laptop - 300-350 RAC
T7400 laptop - 1100-1200 RAC when it was crunching; right now it's folding for the contest
The RAC in that thread that donuts linked are off somewhat as they adjusted the multiplier (or whatever) since then and dropped everyone's RAC down some.
