Copied from the other thread, for emphasis: Firstly, we don't have enough information to accurately answer the question. What games do you play? I have a very similar setup, X4 965 @ 3.8-4.1GHz and 2x GTX 460 in SLI, running on native 1080p, and I'm heavily CPU bottlenecked in many of the games I own. The cold, hard fact of the matter is that you could be running your CPU on LN2 @6GHz and still be CPU bottlenecked against a 580 in some situations (I know, I've tried it).
But if you tell us what games you play, we can make recommendations as to how to optimize your play experience And don't get me wrong, the bottleneck will rest squarely on the 580 in many situations, where it should be
So, based on my experience:
1)Sometimes, but you should be pretty well balanced.
2)Get a new CPU before adding a second 580