The i7-950 is still a fairly good CPU. YES, you will get some bottlenecking in games but your performance will be higher in all of them. I would guess at between 40% and 80% improvement depending on the game. NEWER games tend to scale better.
I have the same card performance wise (Asus GTX680 at 1201MHz) and mostly have no issues getting to 60FPS at mostly MAX quality settings though I wouldn't mind a bit more performance in games like Metro 2033.
The value of that is up to you. If you get 40FPS for example, you likely can get over 60FPS (and lock to 60FPS VSYNC ON?) with the 2nd card.
If you already get 60FPS with a few light tweaks then I wouldn't bother.
Some games like BF4 can be CPU-bottlenecked at times so in those instances...