I'm planning to change my Radeon 9600 with a newer Geforce 7300GT; however, I worry that this exchange will result in conflict with the old files left by the ATI Catalyst Driver (currently using ver 8.4), since I've heard that when exchanging graphic cards from ATI to NVidia or vice versa, there'll be a conflict between the new card and the old files left.

Is it true? Cause I heard that old graphic cards have this kind of problem.

Not so much an issue anymore. Besides, you can use DriverSweep to be sure you got every file.

DriverSweeper (Freeware)
[Only registered and activated users can see links](Setup)_d1655.html

Be sure to download the latest Nvidia drivers, found here; [Only registered and activated users can see links]