My rec. is that you keep your P3 with the Asus and get a new board for C2.... It's got more room for good OC loving fun.... But regardless of which CPU you use you'll be looking at the same handful of boards since there are only couple that are really good for OC.
From my experience the good ones are Abit BF6 (you'll need buy a good slocket and a ATA100 controller), SE6, Asus CUSL2, MSI 815E Pro
BE6-II have some compatibility issues due to onboard ATA66 controller, BX133 RAID limits your Vcore @ 1.7V when using C2 (this can be bypassed by hardwiring the vcore to 1.85 but that's a little too high), CUBX is horrible when it comes to C2 566 OC
I really don't have that much experience with Via chipsets but according to my brother they overclock pretty well too. I think CUV4X and the other slot 1 Via board from Asus are the good ones.