Integrated? I thought they were all based on expansion cards nowdays, one more reason not to get a mac then if thats the case
Definitely GPU overheat.
Pull the card out, give it a thorough brush to get the dust out of the heatsink fins (its the leading cause of overheat in my experience) and then re-seat it. Sometimes the card has to be physically removed and replaced and it will come back to life.
Also when its on, check to see how fast the fan is spinning, if its sluggish it might need replacing with an aftermarket cooler or at least a squirt of WD40
As I said, it's an iMac. is it still possible to open it up and look at the GPU?
Also, would overheating be responsible for me being unable to boot up windows or mac?
The overheating MIGHT be an issue depending on how long you've waited. There is a chance that its dead since a computer won't boot without a GPU. It might also be worth pulling all but 1 stick of ram out and testing them individually since damaged ram can stop a computer booting
New Vegas is so buggy it will actually kill your graphics card. The only way they could make that game more unstable is if the disc shot out of your CD drive and decapitated you when launching the game.
Or we could keep the discussion in the realm of sanity and agree that software cannot kill hardware without access to its firmware which FNV sure as fuck doesn't have