Since you're an overclocker, I'd say you need to get closed phones, the more closed the better. I'd reccomend going with the Etymotic ER4S, since they would provide about 20db of isolation. You will need to try to keep your watercooling and GPU/case cooling as quiet as possible, though, to really allow you to get the most out of a $700 system.
You will need an amplifier for the ER4S, and there are a fair number of options. You could go for tubes (X-Cans, HG Head) or solid state (Headroom Little, Cosmic, Corda HA-1, OBH-11), or have one of the members here make you a META42, which I found to be a very good sounding amp, able to hold its own against the Headroom Cosmic. The META is a portable amp, with many different setup options, depending on your budget.
Since you plan on use these phones for gaming, you might consider it important to be able to use the crossfeed feature that some of these amps offer. That is, they "mix" the left and right signal to give a better stereo image. The Little, Cosmic, Corda and META42 amps can all do this.
For a source, I would say get a dedicated CD/SACD player. Use the soundcard you have now for gaming and MP3 playback (which will start to sound much worse with your new setup), and get something like the Sony DVP-NS500v as a standalone player.
Do search for info on all these products, especially if you are not going to be able to listen before you buy. I would also reccomend buying from
www.heaphone.com, as they are a great company and offer a 30 day return period.
So, if I was you, I'd get:
Etymotic ER4S $269 (
www.headphone.com)
META42 $250 (ask in the DIY or Marketplace forum)
Sony DVP-NS500V $150 (find at a local CC or BB)
Outlaw Cables $ 40 (
www.outlawaudio.com)
Shipping $ 20
Total $739