Is it harmful to a GPU to push it to its limits for hours at a time?

Also I've heard that the cost of electricity can make mining unprofitable if you don't get a good hash rate (I have an Nvidia card so I get about 15 hash/s), but I usually leave my computer on all the time anyway. Is there a big difference in power consumption when mining versus not mining, or playing a 3D video game?

Thanks.