I would not get the 4GB version. I had the 2GB until yesterday and I maxed out all games with awesome fps. I even OCd and got even better. So I decided to SLI it and go from there. Now I'm on Cloud 99999999999999999999999999999999999!
The GTX 670 is borderline for 1040p gaming, but ok for now. In a year though you probably won't be able to max out every title and obtain decent fps. But the 4GB of VRAM do make a difference at this resolution so that's a good idea.
A GTX 680 would enable you not being forced to upgrade any time soon.
The "GTX Titan" will cost a lot more and won't be part of the new generation product-line (700 series), so availability may be very bad.
my buddy had 2.7gb vram on 1440p, i had on 1080p in skyrim castle draco mod 2,55gb vram usage on 660ftw 3gb
who knows some people say 770 will be 3gb card, but hey they were supposed to come in2-3 months people are now saying second half of year towards end of year
Ok, what I thought was wrong indeed. The video card will be bottlenecked by the GPU itself before it can take advantage of that extra memory. So... there is really no point in selling these things then!