I'd say Sennheisers or Grados. I own Sennhesier HD570s and I love em. They're about $150 new or about $80 in-box on ebay, and I think one store was having a sale on them for $80 online. My Senns are
extremely comfortable and I can wear them for hours on end and not get a headache. They're very light and very soft. They don't need an amp to drive them, either.
If you're looking for a crazy sick setup, run over to
HeadAmp and buy a Gilmore amp, then get a pair of
Sennheiser HD650 headphones. Then get a good sound card, like an M-Audio Audiophile with RCA outputs or a Creative Labs Audigy2 ZS (I think you'd want something like a DAC to get the best quality output of the Audigy's optical port).
Keep in mind tho, if you're going to be listening to stuff like MP3s and games only on your computer, you may not want to blow that much money. Higher-end equipment will bring out flaws in recordings and compression issues in MP3s and stuff like that. I'd say buy the Sennheiser HD570 headphones, then if you want, get a small amp such as a
HeadRoom Total Airhead or Total Bithead.
I'm currently running Sennheiser HD570s off my CL Audigy2 ZS and I'm quite happy with it.
If you want some other recommendations, hop on EFnet on IRC and join #headphones.