Overclock and buy a 6800 GT! Even though the 9800 Pro is a really nice card the 6800 GT really outperforms it. My 2.8c will only do 3.3 but you might be able to get to 3.5 if you have a good chip.
But I've have to agree with wisdomtooth though if you do want to go the new mobo/cpu route. You will get gaming improvement and can throw a 6800 GT in 6 months from now when they are cheaper.
Right now you'd get more from a video card upgrade, but this path will also cost more.