ya, I upgraded to my current one from a 9700pro which was what the 9800pro was based on. (design-wise) Both are great cards, even today.
and you will see some older benches showing the 9800pro soundly beat the snot out of the vanilla 6600. In many tests the way
older 9700pro kicks it right in the junk. Doom3 being one exception, but ati has vastly improved openGL performance in their drivers so that is not accurate now. (rally, none of them are as they all have changed I'm sure, but relatively speaking the 9800pro is the better bet)
If you are gonna sell soon and dont want to wait to save for something more like the 6600gt or even better the 7600gs (now in agp) then go w/ the 9800pro