Just a little feeder off my head:
i5 - i7 both have the same number of Cores, except that i7 has 2 threads per core makes 8 Cores. i5 has only 1 Thread per core, make it only a total of 4 Cores.
I currently run a 2nd Gen Intel i7 2860QM @ 2.5GHz but turbo runs it at 3.5GHz, with 8GB DDR3, Blu-Ray Burner, HDMI, 9Cell Battery (6hrs running with VM or 8hrs w/o)and SSD 256GB. I am way more then happy with this. I got it refurbished via eBay. Everything worked and got more bang for the buck as they were worth $2800+, got it for nearly half its price!!!
For me, I do a lot of VMWare and simulations so I required the extra cores and RAM.
It all boils down to what you want to do and what's your plans or goals?
Remember, 3rd Gen IVY Bridge is out. You can learn more about IVY Bridge here: http://mintywhite.com/more/hardware-more/ivy-bridge/
For the average user, I would say, i5 is more then enough, unless you're a hard core user like myself that uses all its available power, RAM, SSD, etc... I am more then satisfy.
Again, let us know what is it you want to do.