I installed a GeForce 6600 in a friend's linux machine last night. Here's all you have to do to install an nVidia driver (as opposed to an ATI):
1. Boot up in "maintenence/rescue mode".
2. Set the GCC compiler to the one the kernel was made with (eg "export CC=gcc-3.4"
3. Run the setup: "sh NVIDIA-blah-blah-blah.run"
4. It takes care of removing old modules, configuring your x config file, building the new modules, etc. It's easier than using the packaged ATI drivers even.
5. Restart: "reboot now"
Upgrading is a snap, just go through the process again. As I said, it takes care of old modules for you. ATI drivers on the other hand require you to make packages, remove the old ones, install the new ones, dick with aticonfigure/fglrxconfig (depends how old the drivers are), screw with restricted libs, and pray it will work...because it won't half the time.
What do you get for your efforts installing an ATI driver? One of two main possibilities:
1) No 3D support (MESA still being used).
-or-
2) Buggy 3D support that's slower than crap.
What do you get for your efforts installing an nVidia driver? Really, only one result:
1) 3D just like you would have in Windows with an nVidia based card.