We really need to know the rating of the PSU to give firm advice. Open up the case and you may be able to see on the PSU a sticker with manufacturer and rating of the PSU.
A card like a 6600GT specifies a 300W and the 6800 types 300 + ( I am an Nvidia type)
A machine which was mid/top end about a year ago will perhaps have a 300 or 350 watt PSU.
With your current specs the card mentioned above are going to be way over what the rest of the system can handle. (no disrespect intended)
Perhaps look into an FX5600 on ebay there are a few and they don't need an additional power supply.
Question still remains about what PSU you have