link82, I wouldn't risk it. If you want a better card, get the new x800, actualy beats the 9800xt by a large margin. If you reallt want to, here is an artical on overclocking the 9800pro.
http://www.pppmail.com/9800pro/
Just keep in mind that you gfx card is very likely to burn out after a couple of months of playing over clocked, or at least overclocked enouph to see a difference. So unless you plan on buying a new one soon, or can spare the couple hundred dollars, I would not recommend doing this.