As I understand it, the iPhone won't drive high-end headphones with high impedance very well. You'll want to look for headphones with low impedance or get a portable amp. I have a pair of the B&W P5s that are specifically targeted at mobile devices like the iPhone and I love them.
I would also recommend that you check out head-fi.org. However, the advice you'll find there will show that Skull Candy and the Dr. Dre Beats are NOT audiophile quality. Also, your price range will not get you true audiophile quality headphones, unfortunately.
Sent from my iPhone using iCafe app