Well, grabbing a second 460 is going to be the most economical approach to getting a big boost in performance. Although, I always prefer single card solutions (the deal on my second 560 was too good to pass up), so if you're willing to put down a bit of cash on top of selling your 460, grab a 570 or the like.
EDIT: If I remember correctly, 460 SLI is somewhere between a 570 and a 580, on average. That's why you'd need at least a 570 (or equivalent) to justify the price, IMO.
Also, congrats on the monitor. I'm jelly