Like with the others are saying here, you're looking at a cascading effect with what you are looking for.
That said, I'm in the same boat as you:
Gigabyte GA-Z77N-WIFI Mini-ITX motherboard
8GB DDR3 PC16000 memory
Intel Core i5-3570K CPU
Sapphire AMD Radeon R9 290 3GB PCIe video card
If I were to upgrade the motherboard, like you are considering, that may require a CPU AND a memory upgrade. Most have jumped from DDR3 to DDR4 memory, so if you find a motherboard that doesn't, you would be okay as far as that goes. Otherwise, you'll be looking to buy the same amount of memory you already have again. Also, depending on the motherboard and everything else you have, you could be looking at a PSU upgrade as well, so it all will start to add up..
Right now, there isn't that much that has changed as far as the sims go, so you should be plenty fine with where you're at. I mean, I'm running P3D 4.2 with a fair amount of sceneries without any problems (Flightbeam, FSDT, and 1 or 2 from Imaginesim, only 2 real payware aircraft - FeelThere and Virtualcol, Nav data from FSTramp and Navigraph, ActiveSky), and am getting smooth framerates throughout.
The Core i5 was released in Q2 2012, so for it coming up n 6 years old, it's fairly resilient. I only went with it as I could easily turn this box into a Hackintosh by simply doing a reinstall of OS X, as it worked with a Core i3 I was mucking around with. But it's a workhorse. Is it worth it? unless there is some seriously major software update/release that would require it, I'd say to stick with what you have. it won't be any slower than what you currently have unless aforementioned update happens.
BL.