Hi,

I have a PC that is running Debian Etch 64-bit with a Geforce 7900 GS PCI Express card in it. Last night when powering up the system the screen stayed blank and I got a BIOS error code that there is a bad graphics card. I am having my brother ship me an ATI PCI express card that he is no longer using so that I can check if it is the card or the motherboard slot. I just thought I should check if there is anything that I should be prepared to do to get 3D graphics for this ATI card to work?

Depending on the card model, you might need to get the proprietary driver for it to do 3D acceleration. You can download it from http://ati.amd.com/support/driver.HTML

Hopefully it has improved since the last time I attempted to install ATI's proprietary driver (in fairness it has been almost 5 years). Do I need to remove any existing files first? The computer still boots and I am able to ssh into it.

Well, it has its high and low points. But, it mostly works. Also, some cards are supported by the opensource drivers (radeon, radeonhd and ati), and it looks like soon all or most ATI cards will be supported by free drivers. Check, which card you have first.
Well, probably, you'd want to uninstall proprietary nvidia drivers if you have them, but I don't think it's necessary. What you will definitely have to do is to edit your /etc/X11/xorg.conf file. Actually, the ATI driver installer does it for you if you want to, at least to some extent.

When you uninstall the nvidia drivers make sure you PURGE them, if you used the Drivers supplied by Debian.

aptitude purge nvidia-glx

I used the proprietary Nvidia drivers actually. I'm not too sure how to uninstall them, but I will look into it. I wish I had another nvidia PCI express card just laying around, but I don't