If you're at 1920x1080 or lower, there's no reason to get more than 1gb. If you're on more than one monitor, go for the 2gb.
Unlocking a 6950 to a 6970 (different from overclocking; don't confuse them) is often successful but sometimes leaves you with an unreliable card. Also, some manufacturers have caught on and have made it physically impossible to do the unlock by adjusting the circuits. It's not a sure thing.
The 6950 would definitely give you a big boost, assuming you have a processor that can handle it and 4gb+ of RAM. What are your full specs?