ed3r, are you using that case PSU? I still suggested to buy an actual brand-name power supply, but good luck with it. I don't think you will have problems with it since your hardware isn't demanding insane power, but it just seems to be an unnecessary risk.
OpenGL VS DirectX:
ATI's perform drastically worse than nVidia in your Doom 3 and OpenGL based games. However, nVidia is slightly behind in DirectX based games over ATI. I may have the DirectX/OpenGL reversed; feel free to correct me.
So what does this mean? nVidia obviously has developed a better card overall. Although both xcompanies have their strengths and weaknesses, nVidia's suffering performance is practically fractional whereas ATI's suffering performance is pretty noticeable.
As to answer your question j4ckaL, if your ONLY going to play those OpenGL rendered games, you would think that you may want to get an ATI.
In my opinion, an nVidia card is overall performance. It's not like because nVidia performs less in DirectX that all of a sudden you can't play the games. I own an eVGA 6800 GT, and I can easily play any DirectX based games at full blast settings with no fps decrease.
Keep in mind that most of the benchmarks between these two cards are at some ridiculously high resolution with some craziness of AA & AF. Unless you will be running those settings, the difference in framerate is incredibly miniscule to be honest.
And when you look at those benchmarks, don't look at the 6800 GT. Look at the 6800 Ultra since a GT is so easily overclocked to an Ultra speed.
I would go with the 6800 GT even though you mainly play HL2 & CSS. It is basically a 6800 Ultra without any problem. You will still be able to play your HL2 & CSS with full blast settings with no problem whatsoever. On top of that, you can play any other game.
Unless you feel like getting 75 fps instead of 72 fps when you won't even notice the difference, I still insist on the better overall solution to suit everything, not a majority. The 6800 GT is what you want.