I would stick with the PC350's for what you are looking for, really they will be your best bang per $.
They are great can's for what they are, and in the future when you upgrade your video card you can search for a proper one with an amp OR find a affordable amp and upgrade your sound. The difference will astound you when you upgrade BUT in the mean time the PC350's work just fine off of a standard pc sound card.
7.1 headphones are cheap and gimmicky, and honestly a waste of cash/time.
Also the option to wait for:
NCIX FORUMS - Sennheiser HD 558 & HD 598
But those are open air cans, so not what you were looking for, my bad!
Still a nice preview!