Well, for web browsing and video watching all you need is a Pentium class CPU
For 20-30 tabs, all you need is RAM - unless you've got 20-30 tabs of video running. Open up your task manager and then open 20-30 tabs in a browser - take a look at the CPU and RAM use.
I don't know how you define light rendering. What application is it? What makes it light - you only do it occasionally?
Take a look at comparable CPU benchmarks
and see how much horsepower you get for the $'s you spend on the upgrade. For grins, the link compares your i7-920 (stock speed) to the i5-4690K. The Haswell i5 is about twice as fast for video transcoding. Going up to the i7 brings a 15-30% additional bump over the i5.
Fallout has never been an intensive resource game, Witcher 3 doesn't use more than four threads and performance is flat above 2.5GHz, BF4 is happy with an i3.
On the GPU side, Witcher is a pig. 1440P with Ultra settings leads to 38-48fps with a GTX 980..and ~73 fps in SLI. It's the most demanding of the listed games. So, that fast refresh of 144Hz probably won't be used until maybe 4 years from now when GPUs catch up...or if you dial down the quality settings. BTW, all of this data is from tech websites that have benchmarked the game. Just use search terms like "BF4 benchmark".
Hybrid cards: do you mean fan control where it's passive for 2D operations and then fan turns on with 3D load? Most of the vendors have bought into doing it this year based on Asus Strix introduction last year with the feature. It's the best thing they've done in years. That said, they all have different coolers, fan profiles, so there is some variation in temps/noise.
Overclock: no definitive answer. Remember that CPU power used is proportional to (Clk freq)*[(core voltage)^2]. So, a 10% increase in freq means 10% increase in power (heat to dissipate) while a 10% overvolt means about a 20% in power. Given your apps, the CPU isn't in the critical path. The GPU is.
You ought to take a look at SPCR's Gaming build articles for some insights to see how much heat can be removed quietly from a case.