Check this link out:
http://www.tomshardware.com/2007/10/01/the_best_gaming_graphics_cards_for_the_money/
And some more extensive benchmarks here:
http://www23.tomshardware.com/graphics_2007.html?modelx=33&model1=854&model2=724&chart=318
Well, I'm not sure how much prices may fluctuate around your area, but here's an educated guess.
I think the most powerful cards you can get for up to 200 bucks are the Radeon X1950PRO or X1950XT, or maybe one of the higher end GeForce 79xx cards (not sure on their price, and some models are weaker than the X1950XT)... but that's in terms of sheer performance.
What I mean is that those are one generation older than the new GeForce 8600GTS, which has a bit less performance... but some extra features (that you can check at nVidia's site) and full DX10 support, in theory. So if you care more about DirectX10 and extra features than performance, this may be the best choice for you instead. ATI seems to have no DX10 cards around this price range, this falls in between their HD2600XT and HD2900 cards.
Also, the X1950XT (and possibly the GeForce 79xx series, not sure on these) require a higher end power supply than the 8600GTS... something like 450W with 35A on the +12V rails compared to (according to nVidia's website) 350W / 25A for the 8600GTS. So if you don't have a good power supply and don't want to spend too much to get one, again, the 8600GTS might be your best choice even though it can't outperform the other (older) cards.
Finally, be sure that whichever card you decide on has DDR3 or DDR4 memory - because most of these models I've mentioned can also be found in versions that have the same names but use DDR2 memory, which is supposed to be significantly slower.
I've been trying to upgrade my system as well, but it's harder to find these things for a decent price around here... I might have to settle for the slower 8600GT or HD2600XT... the 8600GTS and even the X1950PRO seem to cost twice as much no matter how hard I look for them. But oh well, at least the research I've been doing has come in handy already.