Well thats really a dilly of a pickle. And it's not so much of the MB & CPU, but it's really your MB and the RAM you get. If you want the biggest potential overclock go with a P4c 2.4 Retail Boxed CPU. I've seen many with the 2.4c get up to the 3.2 range with Air Cooling and even a few go up to 3.4Ghz with some beefy Heatsinks and some very good case ventilation ( but thats not very often). If you get a 2.4C and a good board and good RAM (which is VERY EASY to do) its a safe bet you'll OC to at least 3Ghz.
As for motherboard, the best one's to go with are the: ABIT IC7 (for about $125) & the new IC7-MAX3 (which runs about $220, but has OTES). Or the IS7 or IS7-G. Also take a look at the better ASUS 865 or 875 boards. Read a few motherboard reviews of the above mentioned, and then settle on the one you want.
But your overall success I think will mostly depend on what RAM you get. I would offer a few suggestions, but really, the choices are endless. Read reviews of Kingston HyperX or Corsair. I'm not really a big fan of Corsair (as it's very overpriced, not saying it's not worth it though.) or PC4000 ram. As with PC4000 ram your paying a big premium and your going to be running the RAM at very slow timings.
Hope this helps, if not PM me.