the k means unlocked multiplier, if your going to overclock get 4670k if not get the non k version, but most if not all people are going to tell you to get the 4670k, and I recommend it too, you will love it.
General consensus, Yes. But it primarily depends on what graphics card you have, and what CPU you currently have, you may be able to turn up more settings and notice a difference. What is your current graphics card?
Oh yeah! Lol, you'll be able to fit a Beefy GPU in there! Have fun with your new rig! Also, I believe the r9 280X can run in crossfire with not only another r9 280X, but a HD 7970. So later down the road, if you want a second GPU and AMD works out the kinks in their drivers, you could save yourself a good bundle on a second GPU.