I had the P4P800 actually I still have it sitting in a box since I switched to AMD.
Anyhow it overclocked my 2.6c to 3.4 at 1.75v with 2GB DDR520(mushkin pc4000).
100% stable and never a lick of trouble. I simply upgraded to PCI-E and decided that rather than buy all new memory (new P4 needs DDR2) as well as mobo/CPu/Video, that I would be better off going with AMD.
A little off topic there, but I did like my Asus P4P800. I had the Abit IS7 but the board I got didn't post and the chipset fan died. I think the Northbridge chipset was shot at that point. When I RMAd to Newegg they had no more IS7s in stiock so I paid a little more and got the P4P800 which was comparable in features.
Back on topic I think either board will overcolck well.