I have a 1GB GTX 550Ti, it runs crisp and clear on my 40 inch monitor. I don't think you should have any problem. But if you want to run more monitors connected to your PC, get the 2GB card.
The downside of the 6670 is that it you probably won't get a high framerate on ultra or even high with your resolution.
I think you should consider a better card such as the 7770.
a 550Ti and a 6670 is a looooot of diffrence...
And yes, I should suggest a 7770, if not a 7750.