Hi Jawa,
I've been doing a bit of research on which headphones you should buy. Fairly sure that Sennheiser is your best bet for what you want them for. There range is large and they are very comfortable to ware for long periods of time.
I've decided your best bet is, instead of buying a REALLY expensive pair of head phones and using onboard sound, is to buy a decent pair of headphones and also buy an Audigy to get the full range and least amount of distortion.
Onboard Audio will create a lot of hiss while listen to music or playing games. You will also loose High's and Lows in your audio which distinguishes between cheap headphones and good headphones. So ether stick with the pair of headphones you have and use your onboard sound, Or grab a Creative Audigy (cheap version will be fine) and use a decent pair of headphones.
So Anyway! Down to what i've decided would be the best thing to get.
First off headphones.
Sennheiser HD215 Has a full range of bass and treble, which also allows long uses with out the need for a break.
The Cord is also Detachable, allowing replacement of the cord if anything nasty happens to it.
Anyway Many good reviews out there, and at the price of $130, they are quite reasonable. (sounds cheaper in pounds though. 33 pounds).