that works either way, famous. if you're replacing an nvidia with an ati, or an ati with a [$alternative_gpu_company], the drivers for the old one need to be removed so the computer doesn't go looking for it.
default drivers will (should) give you enough functionality to find/install the "real" drivers. i think all windows os'es since win2k support 800x600 res on just a generic "vga" driver.
and for a little advice, regardless of which brand you end up using, i suggest going directly to the GPU manufacturer and downloading the reference drivers, not the card manufacturer's drivers. the CDs tend to come with annoying "utilities" that never get used and are frequently poorly written :p