is AMD Radeon HD 7950 - 3GB better than NVIDIA GeForce GTX 660 Ti - 2GB - EVGA Superclocked - Core: 980MHz?

If you're running dual/triple monitors, yes it's better. That's the only situation where the 3GB of VRAM actually makes a difference. Even for a 2560x1600 monitor, 2GB of VRAM is plenty.

On pure speed, the stock Radeon HD 7950 beats the stock GTX 660 Ti by a small margin in most games. The Superclocked version makes up that difference, which would make them a toss-up outside of multiple-screen scenarios where the 3Gb card is preferable.

Here are the latest benchmarks with AMD's version 12.11 Catalyst driver:

http://www.techpowerup.com/reviews/AMD/Catalyst_12...

Of course why buy the 980Mhz Superclocked version when you can get the FTW version?

http://www.newegg.com/Product/Product.aspx?Item=N8...

Of course if you overclock the HD 7950, then it beats an overclocked GTX 660 Ti.

Nvidia cards do have the advantage of better/more stable drivers and hardware level PhysX support, so it depends upon what's more important to you. Purely from the standpoint of highest fps, the HD 7950 wins.