For Windows the difference is minor, some swear by ATI others NVidia. In the world outside of Windows there is no comparison NVidia has better drivers and they upgrade them cosistantly. ATI does not make good drivers for Linux (the only other OS they support is Mac and even here the driver does not seem as good as for Windows). I install both Linux x86 systems and Windows systems for end users and I find that I have the least compatablity problems with NVidia. So if you do not need the graphics card for a specific program go with the NVidia for Dual Boot systems and with the ATI for Windows or Mac only installs.
