There are issues when going from NVIDIA to ATI. The add/remove program does not remove all DLL files from your old drivers and they cause clashes between the 2 cards. However, there are many video card driver removal programs out there (such as detonator destroyer) which do a complete nvidia uninstall simply do a google search.
Once they are safely removed, shut down your computer, install your new card and reboot. Install the latest ati drivers (4.9 are out) or the omega drivers (which are great by the way) and you should have no problems.
My gaming machine:
P4 2.8 GHz
2 X 512 MHz PC3200 in dual DDR mode
Intel D865PERL MOBO
integrated 6.1 MAX audio
Logitech duo MX mouse and KB
ATI RADEON 9800 PRO