p4s are multiplier locked, so you overclock by increasing the fsb
only the northwoods (512 cache) are decent overclockers
the best bang for the buck is still the 1.6a which are 100 fsb chips - with a decent mobo, you will get a minimum of an oc to 133 fsb. many people get 150 = 2.4 ghz
for the best oc boards, take a look at the cpu database at
http://www.overclockers.com and see what they are using
not sure about the celerys, but as long as you are dealing with a 100 fsb, there is overclocking headroom
1 have a 1.6a @2.25 on a msi 645 ultra C using pc 2100 - it runs at 2.4 but is not stable. the highest voltage i can get on this mobo is 1.55