I'm satisfied with the build I have now. Depending on how much your upgrading for, and how much you will be spending on this upgrade, you might want to do it.
The 9800gtx will have a better resale value then the 8800 gts 640 and is obviously better also. So if you have a good sale price on that thing now, then I say do it.
Otherwise you probably still can play all the games you want.