I currently have a Gigabyte mobo, with the old P55 chip, and an overclocked Lynnfield i5 750 @ 3.8GHz (200x19). In one of the PCI-E x16 slots I have a GTX 670 2GB. But since I have a separate sound card for my far above average home-cinema demands using the other PCI-E x16 slot (only running at PCI-E x1 ofc) the GTX can only run at x8 (didn't know that would happen until after installing the card). There isn't space enough to solve that any other way, my VGA cooler is too big to use the lowest x1 slot for the sound card. And the mobo has completely useless old PCI slots at the bottom... Also the PCI-E slots on the mobo are 2.0, not 3.0.
Been thinking about upgrading to more current gen stuff, apart from the GPU and memory (Corsair Vengeance 1600 CL9, 4x4GB) , which I'm both very happy with. And I don't want a multi GPU setup, tried that a few times and just got tired of flickering and many other issues.
So it all comes down to if upgrading now would make any real difference for me. The only demanding thing I use my PC for is games, and I only use one monitor, at 1920x1080. It is 3D capable though, so I do use 3D in a few games. In most games I use maximum on most settings or just under. Games like BF4, Assassins Creed IV, TESO etc run smoothly at very close to (or at) maximum settings. A few games that need a bit more tweaking and fine tuning for me are Tomb Raider, Metro Last Light, CoD: Ghosts and Arkham Origins.
Would going to a Haswell i5 4670k and Z87 chipset in itself make a difference in games? What could I expect to gain?
And would the switch from PCI-E 8x 2.0 to 16x 3.0 make any noticeable difference for my GTX 670?
PCIE 2.0 to 3.0 will not affect the performance of your GPU. Your GPU or pretty much any other one out there atm doesn't make use of the extra bandwidth that 2.0 to 3.0 offers (to be sure to be sure x8 to x16 same biz).
Going to the Haswell you will see improvement in games that are coded/weighted/intensive to CPU. A lot of game logic going on, which I think does apply to the games you play. Your rendering and smoothness in graphics however will remain the same with the same GPU. So to summarise if you go to the trouble of a new motherboard and a new CPU, your gaming will load quicker but will generally look the same. Not really worth it since you're getting close to max as it is. IMO your CPU is still good to go.