I would expect better from the addicts in the community. 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
Just curious, hypothetically, if I swapped out a PII x4 980 BE (3.7 GHz) for an Intel Core i5-2500k, with the GTX 580, how much of a performance improvement will there be?
Difficult to say. The higher the resolution the more the performance depends on the graphics card. Of course if the game is more dependent on the CPU rather than the video card, then having a stronger CPU to begin with will give you better performance.
See following link for gaming performance with various CPUs at 1680 x 1050 resolution. Of course gaming at higher resolutions will result in lower frame rates. It's pretty easy to see that Intel's current CPUs and beat AMD's current CPUs. http://www.xbitlabs.com/articles/cpu/display/core-i5-25...