Is it harmful to a GPU to push it to its limits for hours at a time?
Strictly speaking, it will reduce the lifespan of the card, but of the ~10 I've ever had, I've only ever had one die before I considered obsolete anyway, and that one was both over-clocked and over-volted significantly.
Is there a big difference in power consumption when mining versus not mining, or playing a 3D video game?
Mining vs. Idling - Huge difference. Assuming you have a 9600GT or a 8800GT (about the performance level you are getting), you are looking at around 80-100watts extra power consumption.
Mining vs. 3D gaming - I suspect that it will be more to mine than game, as mining will keep the card at 100% load whereas gaming will fluctuate. I don't expect this to be a large difference though.