To summarize, it really depends on what you want to be doing. For a more serious effort, running more softsynths and fx and expecting to run the latest software and plugins comfortably, go with the i7. I believe the 47xx and 49xx series are currently hitting the sweet spot of affordability vs performance.
If this is more of a "let's keep the costs under control" hobby for you, an i5 will still let you run a bunch of synths/tracks/fx without much trouble. It's still more capable in most respects than the Q6600 I used until a year ago.
Get a good soundcard/interface, if you don't have one yet.