Well according to benchmarks on anandtech the difference will actually be quite noticeable, were talking an extra 15-25 fps on Battlefield 3
I wouldn't have upgraded if I were you tbh. The extra VRAM will most likely help for future titles, but by then you would probably require a card with more grunt. If you are currently playing at 1080p the 580 will last for a considerable amount of time. I play at 1920 x 1200 and I have never seen any game use more then 1440~ MB of VRAM
This is the thing. I really notice it. Anything lower than 60 FPS I spot straight away, even a few frames.
This is what is making me think I made a bad choice. then I see the power consumption and BF3 benchmarks, and think ' I can just sell my 580' and I think it's ok... and repeat.. LOL
Yeh, if you make £200+ with the 580 then the performance increase in BF3, the extra VRAM and better power consumption doesn't look too bad. If I think about it, it's not really a bad decision and as you said you notice the difference when frames drop below 60, so the 670 will be worth while for you.