That's because IB is based on SB as a revision and die shrink. It's just like how Westmere is based on Nehalem, that's why they also share the same socket. Wait until Haswell comes out, it will guaranteed to be a new socket.
Yes it will be. But IB will be around for a year, so it's not a dead socket like you said.
Edited:
How is am3+ a dead socket, because last I checked it isn't.
The i3 on average is going to be more expensive, taking into considering your upgrade options, motherboard costs, etc. It gets better single core performance but despite "emulating" quad core features it isn't a quad like the 965. It's not a bad chip, and i5 is the best processor for the money you can get if you can afford one, but for someone on a budget I'd personally go for a solid phenom2, because its a solid CPU that should hold its own in the future, especailly since there's tons of stuff out there that takes advanatge of quad core now. And assuming piledriver isn't terrible, you can upgrade into that in the farther future.
Though an i3 isn't going to be a bad choice either. I'd personally just go for the thing that in practical terms performs the same but is quad core and has a tendency to be cheaper, especailly since am3+ isn't yet a "dead socket" like you say it is. i3's just never looked that good of a processor to me compaired to the rather solid phenom2 line. i5's are great but you're paying on average $75-$100 more, so its not really in the budget bracket anymore.
All I can say is you're really wrong.
http://www.bit-tech.net/hardware/cpu...3-2100-review/
If all you do is run synthetic benchmarks then by all means get a PII X4. If you actually use real programs the i3 is clearly the way to go. And if the i3 isn't powerful enough then the next logical choice is the i5.