In short, you'll be fine. The amount of RAM is more of an issue than its speed and there's a constant argument over whether or not higher speed RAM is even useful outside of benchmarks; so I wouldn't worry about it.
RAM is different from VRAM, there might be little but no performance issue. I suggest you splash out ~$40 more to get the GTX 760, performs much better and will able to play @ 1080p for next gen games with better FPS. Performance increase over a GTX 660 is quite big too. If it does bottleneck, just grab another 4GB and plug it in down the road.
4 GB is enough since most games out there are still 32-bit.
There is no disadvantage going with 8 GB if you have the money.
One thing that i notice going from 4 GB to 8 GB is the shutdown time is slightly reduced in windows 7.