I have a GTX 690 and only need a 750W PSU, my other components are an x58 system, overclocked i7 920 (to 3.99), 4 HDDs, a PCIe SSD.
General gaming and benchmarking (Heaven, 3DMark 2011) have no detremental effect on my systems stability. Bear in mind I have older architecture that requires more power than Sandy/Ivy.
The critical point is to make certain you have a good PSU - and one that covers your systems demands, including any overclocking. The one I use is a BeQuiet 750W modular.
I don't think you'll be disappointed in a 690
![Wink ;) ;)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
but don't get one unless you plan on taxing it - now - or in the near future (e.g. triple screen, or 1600p).....it'd be a bit of a waste of money otherwise. +1 (as another option) - as bystander said below - 670 sli is not too far behind and will save a fair bit of money. There are various advantages in the 690 single card design/impact on the system; how significant these are depends on your circumstances (e.g. do you need to save space, power, etc).
Overclocking is very simple with a 680/690 - use EVGA Precision X (works with non EVGA gpu's also) this link gives you all you need to know....
http://www.youtube.com/watch?v=NsdIOVsLRac