You won't be able to get another nVidia and SLI it with that setup, which would have been possible had you gone the ATI way. Nonetheless, it doesn't mean the 460 was a bad choice, it just refrains you from using dual nVidia's in future. You can always sell the 460 (if you're not happy with its performance, which is unlikely) and get a 5850, 5770. But then, you can always save up later, sell the 460 and get a 470 which is an awesome card too!
So it's not a loss if you consider it. There are options, depending what resolution you wanna game at.