If you're happy with it's current performance, don't bother. But here's what you do:

O/C it a VERY SMALL INCREMENT at a time. Like 5mhz. Then flash the ROM. Then test it, play a 3d game or something that might stress the card a bit. If it still looks fine, and there isn't any artifacting (glitchy looking white pixels along the edges of 3d shapes, usually), it's fine, and you can keep going. Eventually, you're going to reach a point where it's unstable, or it starts artifacting. In either case, clock it back a bit. Since the 9800XT already has a fan, it can probably go quite a bit higher. If you're really after performance, you can look into additional cooling such as one of these things:

http://www.xoxide.com/arctic-coolin...silencer-3.html

Installing one of those will allow you to O/C it much much farther. IF you aren't that adventurous, these can help and they're cheap:

http://www.xoxide.com/ramsink.html

I have some of these installed on my geforce4mx, not sure if they're doing anything because my memory isn't even hot... so... >shrug<