First off if you go Nvidia, try for Evga. With a lifetime warrenty and they cover overclocking (except if you physically make changes like volt modding and installing/removing chips which can be reverted back to stock anyway), you cant go wrong. If you go ATI, try for Sapphire. I have never heard of any problems with them and their prices are usually the cheapest so they seem to be top dog imo.
If you are going to mod and overclock, get the 7900 GS. If you are going to do very minor overclocking and basically keep it stock, get the x1950 pro. Regardless, I recommend the 7900 GS hands down because of its awesomly low price and just the fact some simple volt modding (takes all of 15 minutes) can give you a big jump in benchmarks and fps. Also get the EVGA 7900 GS because its $149.99 after MIR
i3 540 @ 3066 MHZ
4GB DDR3 1600 1T 9-9-9-28
HD 5750 @ 850/2400
Antec Lanboy Air