i5 is right below i7 and they are all duo-core so they generally produce less heat than quad core i7s. But there are dual core i7s as well. The other main difference that I know of is the i5 is build on a 32nm die while the i7 is built on a 45nm die, and there's 3MB vs. 4MB/8MB of cache. That's way the i7 are generally hotter than i5.
Below that is the i3 and is different from the i5 in that it does not have Turbo Boost; the ability to "overclock" itself automatically. The i5 does not overclock itself that much though; 266MHz on low end i5, but not sure how high for higher end i5.
If I were to buy a laptop, it would definitely be one with a Core i5. I have a desktop for more intensive tasks.