i would prefer not to overclock cpu . . . i5 3570k
Does . . . not . . . compute . . .
Don't replace your power supply with a 500W if you are going to crossfire 2 cards, though it might work it will be very close to your power supplies capacity, you would have to be very careful about your selection to ensure your 12v lines have the capacity to run your machine. If you are looking to get a new one look for something that is fairly good quality in the 650+ range, at least that way you know you are sure to have headroom available.
What resolution are you playing that you need more performance than a 7850? Crossfire will help in certain scenarios, but depending on the game you may find more GPU power isn't going to fix performance problems. However, for single cards your options are pretty limited in terms of upgrades, essentially just the 7950/7970 or an NVidia 670 or 580/680. There are a few other cards slightly faster, but probably not worth the cash to swap them out with what you already have.