What will happen when I change the video board?

I'm about to buy an AGP video board to replace my PCI card. By default redhat boots in graphics mode (GDM) will I have to fix the Xfree86 conf files in the console or will GDM load a generic SVGA driver?

I once had bad drivers and the X Server kept starting and dying over and over forever...

Also, do I have to re install linux to get the card recognized intead of loading the drivers by hand?

I run Redhat 9.

I think Redhat 9 detects hardware by itself. Just like most popular distros.
I have even switched video cards myself once and mandrake simply recognized it. And so it was with my ethernet cards under Redhat 9.

So as long as your software supports what you are trying to install you should have zero problems.