Any issue with rotating the model or graphics?
In terms of saving, you're going to want to get a machine with a solid state hard drive. That will save you quite a bit of time saving and loading. I would start there as your processor is plenty fast, and you have plenty of ram. The specific issues that you mentioned wouldn't be graphics limited.
If at the end of the day you need more processing power, the i7-2600k would benefit you, if you overclock. Keep in mind the performance from the 980x and a chip like the 2600k isn't much out of the box, so like I said that would require overclocking the 2600k to see advantages.
How large are your assemblies? With your specs I'm surprised that you have slow rebuild times. With the slow saves, I assume you have a 7200rpm drive? So the afformentioned SSD will help quite a bit.
I sometimes do have problems rotating and zooming in and out. It will stick and then lock up. Then I get the Solidworks is not responding box.
There is also the potential that a higher performance graphics card could help, but before you think about changing any hardware, it sounds like this may be an underlying problem with the install or with the specific files. Would I be correcting in asserting that this happens for all of your large assemblies?
You may want to start by resetting your registry key for SolidWorks. A corrupted registry key can lead to performance issues especially the 20-25 minutes you're talking about here. I posted the procedure on a registry edit here http://grabcad.com/questions/cant-find-external-references. See if resetting the registry alleviates any of the issues that you're having.
If you are getting constant regens and rebuilds that is usually an indication of a circular reference. Find and fix that and you should not be waiting for rebuilds.
You have a fast computer, I think you may have other more fundamental issues with how your assembly is built.
Give us more detail on your assembly. Types of parts, how feature rich, does it have a lot of patterns. Do you have a lot of errors in the Feature Tree that are not fixed? Can you post a picture of what you are working on?
What does it appear to be waiting on or stuck on while working on your assembly.
Do you have the SolidWorks approved drivers for your system and video card installed?
There is a well written set of tips here, http://blog.cati.com/maximizing-solidworks-performance/ that cover a great many tweaks that can be made to the OS as well as SolidWorks. I've employed many of them with good results. Pushed them out to my 30+ users as well, and it has made our team more efficient as a whole.