Look at the benchmarks of the 9500 Pro vs 9700. You can see the differences vary wildly by game.
In the case of UT2004 with AA/AF 128bit 9500 Pro is more than 18% slower than the 9700, a pretty big difference. But if you look at Joint Operations and Far cry the difference is much smaller, around 10% slower for 128 bit.
So that gives you a range of how much slower than the 9800 Pro to expect. Look at the difference between the 9700 and the 9500 Pro, then scale the 9800 Pro the same amount and you should be pretty close to where the 128 bit version will be. At least in he cases that aren't clearly CPU limited like UT2004 with no AA/AF.
Again the 9700 and the 9500 Pro are the same cards with the exception of the memory bus width. both have 8 pipes, both run at 275core / 270memory just the 9500 pro is 128 bit and the 9700 is 256 bit. It's a pretty direct comparison to the 9800 Pro 128bit/256bit, just the 9800Pro clock speeds are at 380/340 instead of 275/270.