I think you can get a set of Ultegra wheels for about that much. As for The question of improved or noticeable difference between the wheel set you have and a $500.....well, you probably won't feel much difference at all. But you could get a bit of a placebo effect and the new wheels will have a bit of the old bling factor as well.

As long as you have $500 burning a hole in your pocket I say do it....if it is a financial stretch for you then its not worth it at all.