If it is a question between either the HD6950 and the HD 6850 and you are a gaming 'enthusiast' you should go with the HD 6950 (2GB with the dual BIOS) if you can.
It will certainly depend upon your gaming (rez & games) and how you clock that 555BE. If you feel like it will become an issue an AthlonII quad or PhII 840 'Propus' will give you more cores for roughly the same price -- but won't clock quite as high as the 555.
Be certain to purchase an AM3+ motherboard if this is a new build (like the Gigabyte GA-870A-USB3) at the Egg for $102.
Otherwise ... don't sweat the details too much (and enjoy the ride!)