GTX 690 VS Gtx 680 SLI
The GTX 690 is massively expensive and insanely fast. This card takes the crown as the fastest single-slot GPU in the world.
Let's skip all the nonsense and look at the benchmarks. What is faster, two GTX 680's in SLI or one GTX 690 (source: TomsHardware).
1920x1080 (4xAA) - 2 GTX 680's in SLI beat the GTX 690 by 7.1 fps
2560x1600 (4xAA) - 2GTX 680's in SLI beat the GTX 690 by 4.68 fps
1920x1080 DirectX 11 - Tie
2560x1600 DirextX 11 - 2 GTX 680's in SLI beat the GTX 690 by 3.5 fps
The Elder Scrols V: Skytim
1920x1080 Ultra/ FXAA - Two GTX 680's in SLE beat the GTX 690 by 5.08 fps
2560x1600 Ultra/FSAA - Two GTX 680's in SLE beat the GTX 690 by 3.36 fps
So, it looks like two GTX 680 in SLI barely beats the GTX 690 in every test.
Noise, Power, and Thermal Performance
The GTX 680 idles at a lower temperature than the GTX 690 (even in SLI) but the GTX 690 is virtually the same temperature at full load.
The GTX 690 is a tiny bit noisier at idle speeds but a little bit quieter at full load (41.7 dB(A) vs 42.3).
As for the power, the GTX 690 uses slightly more power than two GTX 680's in SLI.
The performance is so similar down the line that either solution is a good way to go. Getting one GTX 690 now will leave extra room in your system for future upgrades. If you can afford one, go for it. I can't imagine this card having trouble with any game coming out this year