I have had very good luck with overclocking 3 different motherboards all of them DFI not one problem here. Just make sure to check DFI-Street.com and make sure you have them help you set the board up with memory and a good powersupply. That being said DFI Lanparties are not for the noob. They will give you very good results, but they are very picky with ram. OCZ and Gskill seam to do fine with them. Most of the problems with NF4 Lanparties where because the person didn't set it up right.
That being said if you want a decent board for cheap then ASrock are pretty good. As to if they are good overclockers I think it is more of a hit and miss thing. I have heard the new one for conroe doesn't do very good only about 2.4ghz on an e6300, plus I don't think it can run at a 1t command rate for memory. where as the new Lanparties can run with all dimms full 1t command rate.
Bottom line if you want speed and overclocking then get a lanparty (be sure to pick your ram right)
If you want a nice cheap board with the possiblity of overclocking fairly well then asrock is your board.
Hope this helps.