I know a lot of people that can run on high settings with a nvidia 6-series...I myself have a 7300 GS, so I don't know what's the problem...but upgrading your card is a very good idea!
The 6 series tends to struggle a bit when it comes to newer games, a single 6800 could probably run without overheating but I doubt the 6600 is up to it.
As I said I run with dual 6800s working together, it produces the performance of some of the newer cards but as is to be expected can't run DX10.
hm...ghugh has a single 6600 card and had only one bug: The registries were black, but then he installed new drivers and everything worked fine...maybe he had some luck!
The registries were black to begin with on mine too, the standard 6 series graphics card drivers don't support newer forms of shaders. :)
Updated my drivers from the disc supplied with the UK edition of PC Gamer.
If you try downloading the newest drivers from nVidia you'll still be stuck with the problem as it's incompatable with the type of shader that causes the black registry issue.
nVidia Drivers 93.71 works fine though.
I can't help then I'm afraid, my knowledge of graphics cards only covers nVidia as I have a distaste for Radeon.
Although you could try and update your Radeon drivers.