As of right now, 1GB is usually enough. I would not advise on purchasing a 1GB card though, since newer games should quickly go over that limit. That will happen specially faster once the engines designed for next-gen consoles start hitting the PC, which shouldn't take long. A 2GB card is a good investment. It is worth mentioning that skyrim with High-Res mod can easily go over the 1GB limit.
In case you don't know, more VRAM will not make the card faster, it will instead give you more headroom. If you ever go over the VRAM limit, the game will become unbearably slow. Forgive me if you already knew that, but this matter does pop quite often over here.
As for the OC version, it depends on the card. I for one am not very fond of the...