As far as changing video cards from nVidia to ATI, the best thing to do is get Driver Cleaner, Nasty File Remover, Detonator Destroyer.
Then, uninstall the nvidia card using Control Panel-->Add Remove Hardware. At this point, don't restart your computer.
Next, run Driver Cleaner, detonator Destroyer, Nasty File Remover. and a registry cleaner program to get rid of all the little files left behind by nvidia.
Then, install your new card and drivers, you should be set to go.
Intel P4 2.4B MSI 645E Max-U Mobo 1GB DDR333 ATI R9600XT 128MB SB Live 5.1 Samsung 120GB
Maxtor 120GB LG DVD+-R/RW WIN2K PRO SP4