It really depends on what you want the laptop to do. If you were running intense CPU software, like transcoding video, photoshop filters, etc. You might want to just look at i7's. Nowdays some of this (previously mentioned) software is also accelerated to use a GPU, so a laptop with a (NVIDIA) gpu would bring additional value to your purchase.
Additionally, i7's can be quad core, while i5's and i3's can't. If you're at all technical, the table at:http://en.wikipedia.org/wiki/Sandy_Bridge#Mobile_processors
should give you a good enough idea.
Just don't go for the previous generation of i7's (look for a four-numbered model).
Overall, it always depends on how much money you have, and what you actually need it for.