Oboe,
I was in a similar quandry, also upgrading from an E8400, but the Ivy Bridge chips weren't out yet, so it was between a i5-2500K or i7-2600K. It was around the same price difference too -- $100ish.
I went with the i5, simply because I don't do anything that really needs the HT. The most I do is work with very large RAW images (18 MP), and I spent the extra $100ish on other stuff. An extra couple of seconds, literally, isn't going to kill me. If I did video work, I would've gone with the i7.
However, it wouldn't hurt to get it though. You can't go wrong with having 'more' available to you.
Here's a comparison between Ivy Bridge and Sandy Bridge, with some times in various tests (not just games):
http://www.tomshardware.com/reviews/ivy-bridge-overclocking-core-i7-3770k,3198-5.htmlAs far as motherboards, Z77 wasn't out yet, so I went with a Z68 chipset. The actual board I got was a GA-Z68XP-UD4. Research a bit on the motherboards to see what's best and what people are having good luck with.