What's your soundcard? The best cost - value headphones is without a doubt the Sennheiser HD580. That's what attracted me to them. I got mine used for $120 off of the buy/sell forum at
www.head-fi.org.
The HD580's can be driven quite well straight off of the Sound Blaster Audigy 2 ZS Platinum Pro. However, headphones with higher impedances sound MUCH better driven off of an amp. I'm driving mine off of a Headroom Total Bithead amp right now, and it evened out the lows and tightened the lower-mids and mids.
I highly suggest buying used off of those forums - the users there take real good care of their equipment, and you don't have to worry about the burn-in time with new headphones (usually you put new headphones on a table and play music through them for ~200 hours in order to set the drivers into their final position).
I've heard good things about the ATH-A700, and almost bought a pair myself, but the HD580 was right-on for my budget and sound. The HD580 is an "open" headphone, meaning sound is not limited to your head - it escapes very easily and open headphones aren't very good for the purpose of not letting anybody else hear (although the 580's don't leak that much).
Closed headphones, such as the ATH-A700, are great for blocking out sound from other people in the room, but in my experience SOUND closed. The open headphones sound very open and free, and in my experience listening with closed cans can be fatiguing. However, I don't have too much experience outside of my HD580's and my previous headphones, which have all been open, and the few that I've tried out for short periods of time.
If you're thinking about dropping some cash on new headphones and still aren't sure, check out the forums I linked you to. They'll likely suggest the HD580's as a budget/starter audiophile headphone, as they did with me.
