Remember the stock heatsink comes with a pad on the bottom. You need to scrape off that thermal pad if you're going to add your own thermal paste to the bottom. You do NOT want both on there as you'll get worse temps.
If you're serious about overclocking I highly suggest you get an aftermarket heatsink. You CAN OC with the stock heatsink, but for better temps and chances at OC'ing aftermarket is the way to go.
ABit or DFI make some of the best OC'ing boards out there, so I suggest you look to their boards
ABit has teamed up with Jonathan 'Fatal1ty' Wendel to make his own brand of board, one for Intel, one for AMD, that's supposed to be great for OC'ing.
Kevin rose had it on the screen savers and he managed to lock up the computer and require resetting the CMOS....as I watched him I'm thinking "with all this knowledge he has you'd figure he's completely familiar with OC'ing" but yeah he was seeming kind of noobish.
So yeah you could look at the AMD version of the Fatal1ty board, supposed to be all good with OC'ing and has some of the voltage chips with heatsink on it, and fans over the RAM, but yeah, I would suspect with your own cooling the ones such as the AV8-3rd eye or AX8-3rd eye (if you wanted PCI-E) would be as good too.
DFI's Lanparty series is a good OC'er
But you need to READ READ READ READ, I can't stress this enough. Just last time I had 4 threads in here "Help, I jumped my FSB from 133 to 200, now it doesnt work what do I do!?!"
If you've read anything into OC'ing you shouldn't even need to ask that kind of question.
So please, do yourself and everyone here a favor and READ all you can on OC'ings basics...OC'ing using the board you plan on purchasing, the OC'ability of the CPU you get, OC'ing on the kind of RAM you get also.