As LSB and BBGun alluded to, the real issue for me insofar as Aces High is concerned is visual quality, not FPS. I have my FPS limited to my vertical refresh of my monitor, so I wouldn't expect anything above 85 anyway. When it comes to other games, however, I wonder just how much of a performance gain in fps I can get with the ATI over my Ti-4400, considering that the CPU may be the bottle neck.
When I was at the con last year, I remember walking by and seeing everyone's AH screens, and they looked similar to mine. They all pretty much were running GForce cards. Then I happened to walk by Fatty as he played, and I couldn't get over how much more clear and crisp and colorful his AH screen was compared to the rest. When I asked him why his looked so much better, he told me that he was running with an ATI Radeon 8500.
So, maybe it would be worth the money to upgrade just for an improved picture clarity and color pallette. On the other hand, it sounds like if I didn't go for the 9800 Pro, I wouldn't be gaining all that much in fps.
One other note--I just noticed that you're running a flat-screen monitor. When I was investigating those, it seems like I read somewhere that the ATI cards' DVI output only worked properly up to 1280x1024. Perhaps your monitor has an analog input?
My monitor is a flat screen CRT monitor, not one of the new slim flat panel monitors. Your comment about not being able to go above 1280x1024 is exactly why I'm sticking with old technology CRT monitors. I'm not going to jump into flat panel screens until the resolution and refresh rates are higher.