A HD7850 is very low power usage, as graphics cards go. That system will run fine on a 450W PSU. If you are overclocking, and I assume you are, 500W should give you enough to play with.
If you want to cover the possibility of upgrading graphics cards in the future, then somewhere in the region of 600W will cover you for most modern graphics cards.
Remember also that wattage itself is irrelevant, you need to consider the quality of the PSU, and how much it is capable of outputting on 12V rail(s), as that is where the vast majority of power draw on modern PCs comes.
almost the same points lol
few seconds time difference.