Scaling of GPU's any brand (NVidia/AMD), any model falls off VERY rapidly after two (2) cards. Check out any number of reviews. Also, in those reviews, support for four (4) GPU's is almost non-existent in games and buggy at it's best.
GTX590's are out of date & obsolete (as new cards are faster for less money). Not even sure where you could find ONE GTX590, let alone a pair....
As others point out, GTX 780 or 780Ti would be the recommended starting point for a new build....
And while GTX 690's are a current card, they would not be my choice for $2000. Dual GTX 780Ti's would crush them for about $600 less (before you bought any water cooler bits...)
I would never recommend anyone's 1st build is a custom water cooled loop system...