You have to program something for multiple cores if you want it to use multiple cores.
With high end audio and video editing, people have no choice but to use multiple cores if they want the tasks to get done any time this year.
Games, on the other hand, aren't nearly as processor intensive. In fact, they generally try to make games rely on video cards as much as possible so people with sucky processors can still play them.
That is why games generally only use 2 cores while audio/video will use the whole 8 no problem.
An i7 would matter, yes. It depends on which one, but the i5-2500k is generally considered to be near the top of the heap for gaming because it has 4 great cores and no game uses more than that. The 2600k and 2700k and other i7's have the benefit of Hyper Threading (meaning 4 cores act like 8), but it doesn't matter for gaming because few games use 4 cores anyway and absolutely none use 8.