go for gtx 670 sli dont go for an single gtx 690 the gtx 670 sli will give you the same performance as the gtx 680 sli and single gtx 690 and the price is way less you can save upto $200 with getting an gtx 670sli which gives almost same performance as the gtx 690.
look at this review then you will get what i m talking about.
http://www.techpowerup.com/reviews/NVIDIA/GeForce_GTX_670_SLI/4.html