Go for the 670 SLI, it will be cheaper and almost identical performance to the 690. Overclock them and you may surpass the 690.
670 SLI = $800
GTX 690 = $1000+
You really do not need to go with Ivy Bridge , its not a good upgrade from Sandy Bridge.
You would be better off overclocking that awesome i5 2500K of yours and enjoy the speed of it, or go with an i7 2600K and overclock the hell out of that!
Thats my opinion
My vote goes for 670 SLI as well. If you're running that 2500k on stock cooler I suggest you buy a Coolermaster Hyper 212 (EVO or PLUS, doesn't matter which one) and push that CPU to at least 4.2 GHz. I personally run mine at that speed and it doesn't require much tweaking! Good luck