On a $200 budget, if you prefer nVIDIA, get a GTX260 Core 216 model. If you prefer ATI, get a 4870 1GB. No need to spend the extra money on factory overclocked versions of either. Go with a dependable manufacturer and their basic, or "vanilla" version. You can overclock it yourself very easily in the tools provided by nVIDIA and ATI with their drivers.
You could also do what many people are doing and wait a couple weeks or so and see how the release of the new 4890 and GTX270 affect the prices of their sibling cards.
FYI, your 8800GTS 512 = 9800GTX(+) = GTX 250
It is exactly the same as the newer cards just clocked a little lower.
If you want a good, free upgrade, overclock it your self.
If, after you give your self a free performance boost, it is still to slow, you will need at minimum a 4870/GTX 260 to show any further performance gain.
I am also thinking og moving up from my EVGA 8800 GTS 512 mb. Seems when I push my e8400 to 4 GHz (8 X 500 fsb) my old 8800 GTS starts to bottleneck.
I would wait at least 2 weeks to see if prices come down on the GTX 260 - 216 core models. It's a waste to upgrade to the 9800 GTX+ because you'd be spending $150 for at most a 5% performance gain. I wouldn't buy the 4870 either (bad drivers, heat issues, rendering issues - leaving out textures)