upgrading to a pair of 590's would probably result in a long wait, as i presume they are still like gold dust.
As for a suggestion on what to do it is a tough one. It can't be long before Nvidia finally get round to launching the 6xx series but who knows. They will probably come out with the 680 first if the mirror what AMD did, might be an option to hang on a little bit for those. Apart from that i can't see any way to swap to a different Nvidia card as they all have less VRAM....
2 x 580's beat a single 590 as the 590 is underclocked slightly.
It really is difficult because of the monitors you have, waiting for the next gen cards from Nvidia is about all i can think of to be honest.