No worries, anyway according to AMD
a 450W or higher power supply is needed for the card to run in your system. So I'd say a 500W PSU, bung it into your system and off you go.
In terms of bottlenecking with that CPU I don't really know. I suppose you may see a little drop in performance but all in all I think that CPU would be fine with a 4850. If you were comfortable you could even overclock your CPU a little more just to get that little extra drop of performance.