The reason I took the i5 is because core-for-core, the i5 is significantly faster than an AMD offering. Since many games only use two threads, and most can't use more than 4, the i5 is the way to go. (This is reflected in most builds on Tom's.) If you're doing video editing or other heavily-multithreaded tasks, a FX 8350 will be a good choice, but just for gaming, the i5 will be significantly faster.
As for how important the processor is, it's sort of like RAM - as long as you have enough, adding more doesn't help, but if you don't have enough, then you get huge performance gains by adding more. The i5 is that top sweet spot.