I was actually thinking of going to an i7-4770 if I had to upgrade, as I like the hyperthreading in the i7 line.
Based on the usage you've described here, I don't think you'd benefit 40% by going to an i7 over an i5, especially when your Encoding requirements are "has to finish overnight" rather than "need to save x seconds/minutes." (FYI, 40% is how much more the i7-4770K costs over an i5-4670K) But you can look at the review comparisons
and decide for yourself. An i5 is still going to outperform your i7-870....
Gaming = negligable difference between i5 and i7
Encoding = biggest difference between i5 and i7 because of effective multithreading support, but you have to decide if the time reduction is worth an extra $100.
Main: i5-3570K, ASRock Z77 Pro4-M, MSI RX480 8G, 500GB Crucial BX100, 2 TB Samsung EcoGreen F4, 16GB 1600MHz G.Skill @1.25V, EVGA 550-G2, Silverstone PS07B
HTPC: A8-5600K, MSI FM2-A75IA-E53, 4TB Seagate SSHD, 8GB 1866MHz G.Skill, Crosley D-25 Case Mod