Here is a handy tool for you to determin how many watts you PC will need: Antec Power Supply Calculator
, keep in mind though this is and estimate but a good number to go by when looking for a PSU. Since the 6850 is so new it is not included into the Calc. yet so use the 5850 on the list, it requires more power than the 6850 but will give you a good amount of headroom.
From your specs. and helping others with the same question a nice ~600w PSU is probably what you are going to need, here is a nice model to consider: SeaSonic S12II 620 Bronze 620W
. What other future upgrades are you considering?
If you have more questions after using the Calculator, I assume you will, let us know how many watts it recommended for you.