Yes, Steam games run FINE on Vista contrary to what many people say. I don't know about any performance gains because of your vid card however. If anything you'll probably take at least a slight performance hit with Vista.
One word of caution - not all hardware manufacturers have been good about getting solid Vista drivers out. Personally, I've had really good luck, but I've had friends who have not been so fortunate. You might want to evaluate the hardware in your system before diving into Vista - make sure there are good chipset and video drivers out for your system. I know ATI’s drivers have been pretty solid for me, but I’m not sure how NVIDIA is doing.