What about a PowerColor Radeon 9550 card? Those are essentially underclocked 9600s, and bench somewhere between 9600 and 9600 Pro. They can (I guess) be overclocked to 9600 Pro speeds, and should be cheaper than just a straight 9600. However, they were originally intended to be sold only in "price-concious" sales regions around the world, so finding a supplier for them may be difficult.
Even with a faster video card, I have to wonder if perhaps some of the more modern games aren't CPU-limited as well. With only $100 to spend, I might consider putting that into a better CPU instead, and OC'ing it to 2.3Ghz or whatever they do nowadays.
True, that won't get you any better eyecandy, but it might up your framerates, and you would be in a better condition to upgrade your video card in the near future, perhaps if ATI Radeon 9800 Pros drop some more in price in the next six months. (Or they might just not, I can't say.)