[adonys]

3 GB GTX 580 is worse than normal/overclocked GTX 580, as the clock of the graphic/shader processor is lower.

You'll have to chose between them considering other factors, mainly the resolution at which you'll run the games:
- if you plan to use normal resolution (like 1980x1080), and most of/the most important of the game you're playing won't exceed 1.5 GB of VRAM (and I don't know of any atm) -> it's better to go with normal/overclocked 580s. For 1680x and lower ones, even a 560 Ti [super][overclocked] or a 570 would be enough
- if you plan to play on surround resolutions (3x1, etc) then you'll need as much of extra VRAM as you can get -> it's better to go with 3GB 580s

And of course the performance of a videocard is greatly depending on how your system can provide and make use of it:
- processor
- motherboard
- PSU
- drivers
- RAM
- OS