Encoding software tends to take advantage of as many CPU cores as you can throw at them. So if you're going to be doing a lot of video encoding, a CPU which is hyper-threaded (or the AMD equivalent) would be better (eg i7-3770K). If you're going for a system where you'll occasionally encode but mainly game, then it makes little sense to spend an extra £100 on a feature you'll use once in a while, so an i5 CPU would make more sense (eg i5-3570K).