The graphics card was overclocked by BFG tech, which is supposed to be quite a well known graphics card supplier. And it was only very slightly overclocked, and had never displayed any kind of artifacts in other programs such as 3DS Max, Maya, and ZModeler.
When I tried updating the driver, it says, "The best driver is already installed" or something... so I guess it is all right.
So I still don't understand why Rhino's viewports are screwed up.
Maybe I should try updating Rhino..