Black Ops 2 is a game that can be run by any average Computer. It will surely show a difference if you'll upgrade.
1B or 2GB will make no difference on many games but on high end GPU bound games (MOH Warfighter) you require memory around 2GB, and black ops 2 is not that type of game. If you'll only play Black ops 2 than 6670 is good enough.
RAM too doesn't have much effect, even 4GB is enough for Black Ops 2.