Before you buy any headphones, I highly recommend you hear a pair of HD-25-II's.
You'll see them on every sports broadcast (but with an added microphone). They sound incredible, they're small, they fit very snugly on your head (perfect for running for example) and they're comfortable enough to wear for hours both physically and audio wise. I can't hear much of anything when they're on, but they don't actively cancel outside noise. They're well known for cutting out background noise and being able to wear them for hours, hence their dominant use in broadcasting, DJing and audio engineering.
I've owned expensive cans from AKG, Fostex, Sony, and others by Sennheiser, and these are the ones I've been happiest with. You'll never be disappointed in owning a pair of these.
If I ever decide to buy a pair of $800 headphones to sit in a chair in a quiet room with the lights off.. I can see the value in that. But for every other situation, these headphones are the shit.
http://www.amazon.com/Sennheiser-HD2.../dp/B000TDZOXG