If you already got Skyrim, just download the mod and check your performance! It could be better than what I told, coz the 7790 isn't exactly a weak card. It may run at more than 30fps!
And yes, get the extra 4gb ram, as it's the standard for gaming these days anyway.
Before you buy a new GPU though, first check the performance with your existing GPU..
Well I ran it 30-50 with a Phenom II 965 x4 BE and 6670. Your CPU is sort of similar to mine, but your graphics card is better than my old 6670 so I don't see why you should have any problem running it 30+ at all times aside from lots of stuff going on at once, upgrade your RAM as well so Skyrim can utilize the full GB if need be. Hope this helps
And yeah I was running realvision ENB at some high and medium settings.