Hi ToonArmy,
Here is the official system requirements for the HD 4850 graphics card. Note the power requirement. You should be OK with your existing PSU, however you will be running fairly near its maximum rating with very little room, if any, for further system expansion. I suggest that you upgrade your PSU to at least 650 Watts.
(Note: Because this is an image, if you wish to follow the link in green, you will need to type the address manually.)