External headphone amp, cheap and does what it says. Not used one myself but the reviews are pretty much all good.
I've tried the PC350s and they were dam nice, comfortable and closed back, meaning no ambient noise and people around you can hear everything you do.
In terms of soundcards, you really want one to get the most out of the set. For audio look at the Asus Xonar D2 or D2X cards (Same but one uses PCI and the other PCI-E) and for gaming, look no further than a Creative X-Fi Titanium or any of the X-Fi range (Excluding the Xtreme Audio and the low profile Xtreme Gamer, the audio is useless and the Gamer not the best.)
I personally use the X-Fi Xtreme Music (Along with Robbi) and take it from us, the sound difference from onboard is incredible, however you can get better. The X-Fi Prelude is a perfect gamer/audiophile bridge card, just ask Jaymz

