Gaming, it will perform exactly the same until games start to use four or more cores. As for movie making. What programs are you going to use? Adobe After Effects and Premiere, as well as AVID and Sony Vegas Pro are a few that I know, and know that they do demand a lot of CPU power, and can use more than eight threads. If you're doing something big with those programs, and you believe the shorter render times justifies buying another, painfully expensive CPU, then go ahead. If you're going to be using something like Windows Movie Maker, don't bother at all, keep your i7 975 - it's still an awesome.
Ignore the rabble....go and do it. I just swapped out my 975ex for a 980x. The larger cache does make an impact in Windows apps, and rendering is faster. Games are about the same, which is to say - maxed out.
I noticed you are not overclocking your 975 - you will make a significant increase in games and rendering if you increased the multiplier even a little.