If you overclock EVERYTHING an NVIDIA seems to oc much better, at least the older ones, I've not had the sand to try and fry a new video card..... I've read now though the newer model ATIs o/c just as easily as the nvidias...
It's interesting the majority is saying ATI, I'm all the time seeing people selling their 9800 pros for the newest nvidia card.... hmm...
Me... I like ATI, i'm using the all in wonder 9700 pro as well..... it's served it's purpose nicely