Unless you are running 3+ monitors, 2gb of VRAM is plenty for gaming. If you will be doing more workstation-like tasks, I would consider the 4gb card for a bit extra performance.
Consider this post by dannyboy2233,
"I hate to say it, but the concept of "future-proofing" in computers is simply nonexistent. I can almost completely guarantee you that you will start to see the games lagging and your graphics card struggling before you will really need the extra VRAM.
Take, for example, the GTX 295. Released in early 2009, it was the top-of-the-line, dual-GPU card from NVIDIA. Now, only four years later, it is practically obsolete. I can go on ebay right now and buy one for 20% what it originally cost, due to the fact that, when it comes...