A couple weeks ago I happened to be at a certain computer store. I happened to notice that they had a sale on a GeForce 9400 GT 512 meg video card for $40 (normally $70). While I already have a sufficiently powerful video card, I'd been thinking for a while about picking up a cheap GeForce, in case I ever felt inclined to play with CUDA (my video card is an ATI). So, I grabbed one.
After a sufficient amount of procrastination (specifically, two weeks, bringing it to the last day to return it), I figured I should see about opening it and installing it, to make sure it works. But first I decided to do a bit of research: namely, whether you can have both an ATI and nVidia card in the computer at a time. Obviously this should work, but with the ATI and nVidia war, driver idiocy, etc., you never know.
Well, it turns out that you can't - at least, not in Vista (I'm using 2008, which is based on Vista). Specifically, Vista only allows one WDDM display driver to be loaded at a time; this means that if you have multiple video cards, they all need to use the same video driver. While Vista still supports older XP XPDM display drivers (and allows multiple XPDM drivers to be loaded at once), it'll cost you - WDDM drivers are required for things such as Aero and DirectX 10; worse, you can't have one driver that's XPDM and one that's WDDM. Windows 7 is rumored to support multiple WDDM display drivers at once.
And that's why I tried it while it was still returnable, and didn't open it before doing some basic research.