Video cards and support machine

I'm getting a new system and need some feedback on video cards.

I'm looking at the NVIDIA GeForce GTX 670 or possibly going for the 680 2GB or the 690 4GB.

I've heard that the 670 is better because it runs quieter and is less likely to burn out. Anyone know if this is the case?

I really want to get a system that is going to last as long as possible for gaming performance, so I was also thinking about the possibility of an SLI machine with twin GTX 670s. Would this be a good idea, and are there any drawbacks? I'll be running dual monitors as well.

And lastly what type of specs would I need to run twin GTX 670s if it would be a good thing to do.

Thanks all,