It doesnt really matter what you use , per se. It depends on which 2 models your comparing.(GeForce vs Radeon)
As far as switching goes, sure you can. If you switch from Nvidia to ATI you will have to install ATI's Drivers.
If your comfortable with Nvidia then just get a better Nvidia card, which means you'll just have to update your current software on your computer.(I think this is waay easier)
Im Running EQ2 on a 2.4ghz 1 gig ram , Geforce 6800 512mb video card, and i can run the game smoothly most of the time on Very high performance...