well my past experience tells me different. i have swapped out a lot of amd for nvidia and vice-versa and nearly always had trouble with system stability until i figured out a repair install rewrote the hal. (which is a lot easier than trying to do it from inside windows which is the third option, that i purposely dropped because its to complex for most users)
A hardware abstraction layer (HAL) is an abstraction layer, implemented in software, between the physical hardware of a computer and the software that runs on that computer. Its function is to hide differences in hardware from most of the operating system kernel, so that most of the kernel-mode code does not need to be changed to run on systems with different hardware. On a PC, HAL can basically be considered to be the driver for the motherboard and allows instructions from higher level computer languages to communicate with lower level components, but prevents direct access to the hardware.
but because the hal is created at install of windows its set firm in its initial config. so if you first install with an ati card, the hal layer would be slightly different to an identical system but with an nvidia card. its this small difference that causes stability issues. yes the pc will still work and you will still get an image on screen as it should, but you will also start getting random bsods that will increase in frequency until you have no option but to reinstall.
so while "doesnt matter" is technically correct, "it doesnt matter if you just want the system up and running", if you want to use the system after words and use it safely (without fear of loosing data) the bare minimum should be a repair install.