With your budget/usage you can possibly get two headphones which will fit your needs.

 

Audiotechnica ATH-AD900 for their comfort and usage in gaming and instrumental music.

 

Grado SR-60 for when you want to listen to rock music.

 

You can get both for under $300 and both will be fine being powered from your soundcard. No amp needed.

I think PC360's if you need a mic.

Denon 2000/5000s. DAKmart has refurb and if you are in USA just get a cheap 2 year warranty from square trade, saving you money and getting you value.

Denon's are too bassy/muddy for instrumental music. Try maybe Ultrasone's as they are good for rock and movies, and get a nice $100-150 mic.

 

That also depends on the source you use to drive them with. Then again my modded cans are in no way muddy and are more detailed than you'd be led to believe. I cannot stress how revealing these cans are. 24/96 FLAC really suites these cans well.

The HD650s are pretty good at many genres, are very good on the mid range. Still have plenty of bass and are more easy on the ears. You just need a good amp to drive them. They are open cans so noise will leak if that is a concern.

When I think of "all around" headphones, I think of the most flat, neutral, and uncolored headphones out there.  Sure, every headphone adds a little coloration to the sound, but some add less than others.  In your price range, two cans come to mind: Sennheiser HD600 and Beyerdynamic DT880.

the dt880 little hot on the top-end making it sizzy sounding. hd600 is perfect recommendation for an all-rounder and i would throw in the k601 as well. the k601 is very comfy and much better than the k701/q701 in my opinion. the k601 and hd600 might be discontinued but lot of stores still carry them stock and can be found well under 300 and sometimes under 200 especially used.
Edited by RexAeterna - 12/31/11 at 10:08am

DT880 would probably be nice, comfy and with a smooth sound with a good punch to the bass if it´s needed..Get the 32 or 250 ohm version..The Stx would do just fine in this regard.

 

 

It also depends on how much bass do you want? Do you want very tight and lean bass or do you want weight and more punch?

Have you considered the Sony MDR Z1000?  Great all around can that you can get for around $300 from Beach.

Closed, balanced across all frequencies.  Don't know much about gaming but EXCELLENT for instrumental music and movies. 

Give them a few weeks to break in.  Sort of unbelievable what these cans can do...