My guess is that your /etc/X11/xorg.conf is configured to load the "nvidia" driver but as there's no longer an nvidia card in the system, X fails to start and you get the standard debian "X failed to start" screen (can't remember the exact text but it does have a blue background).
Unless you need particular options in X, your best bet is just to rename /etc/X11/xorg.conf to xorg.conf.bak or something and restart your display manager (probably gdm, ldm, kdm or xdm). These days X auto-configures itself quite nicely and a manual configuration often isn't needed.
BTW, to help debug problems in X you can find all the warning and error messages in the most-recently started X server with:
egrep '(WW|EE)' /var/log/Xorg.0.log
However, if you're running a 64-bit OS then it's a bad idea to use cards that are known to have problems with 64 bit drivers. I strongly recommend using a different card that doesn't have such problems. Your original 7950GT was a PCI-e card so that means your machine has a PCI-e slot - that's good because PCI-e is the current standard so PCI-e GPU cards are plentiful, and there are many cheap options.
I don't know exact pricing where you are but here in .au you can get a 1GB AMD 5450 card for about $30 new, or an nvidia GT210 for about $28. The 7950 was a good card for its day (2006) and neither the GT210 or the 5450 are quite as fast but they use a lot less power (~82W for the 7950 vs ~31W for GT210 or ~20W for the 5450) and run cooler and quieter. they also suport newer GPU features like video playback acceleration.
If graphics performance is important to you (probably not if you're only doing 2D desktop type stuff) you can get quite decent nvidia or AMD cards for under $50 or $60.
Either AMD or nvidia would work, but a newer nvidia card would almost certainly just work "out of the box" with your existing xorg.conf.