In most games the HD4850 outperforms the 9800GTX, but not by a huge margin. In other games the 9800GTX performs just a tiny bit better. http://www.anandtech.com/video/showdoc.aspx?i=3341
So far i haven't seen any good benchies where all the cards were overclocked as high as they're capable of being overclocked, or even factory overclocked. If your mobo is having problems with the ATI, then a switch from the HD4850 to the 9800GTX is a pretty minimal one.
$180 for a factory OC cardhttp://www.newegg.com/Product/Product.aspx?Item=N82E16814133220
even the mild OC 8800GTS 512 wouldn't be a huge hit in performance at $160 after rebatehttp://www.newegg.com/Product/Product.aspx?Item=N82E16814143119