Well, your PSU has 370watts available on the 12v, so you actually won't have to upgrade.
TechPowerUp has that model HD6950 pulling 153watts, coupled with your processor's maximum pull of 125 watts..you should be just fine. It's times like this you pat yourself on the back for getting a good PSU.
Keep in mind, however, that your HD6950 could be pushed a bit further, as well as your processor. Should you choose to do so, I would then get a high quality 600W PSU or an average 700W one.