but gtx 560 ti is 1gb and 6950 is 2gb.. will that make any difference for high end gaming?
Only if you game at a res above 1080p (which I'm just guessing that your 18.5" monitor ISN'T higher than 1080p).
1GB of VRAM is just fine for 1080p and below for almost any game that I know of, except for possibly
BF3, and there are PLENTY of people playing BF3 on 1GB cards just fine (including myself).
And before you ask, no there's no performance advantage to having more than you need either. The graphics core itself matters a lot more than the amount of VRAM in most cases.
Also, you WON'T be able to unlock the 6950 to a 6970 like Kamen_bg said. That stopped being possible VERY early on in its production because AMD got wise to it and made it impossible on all newer versions of the card.