OP, I've been doing some research myself on the same stuff lately. After several days of scanning over the Head-Fi forums, the top gaming headphone recommendations I found are the Sennheiser HD 595, Beyerdynamic DT770 or possibly DT880, and Audio-Technica A900. Here's a summary of the word on the street I could find:
The Sennheiser HD 595's are open headphones, so they're comfortable to wear for extended periods of time, but they leak sound like sieve. If you're easily disturbed while gaming or are concerned about disturbing others, these may not be the best choice. These are "all-round" headphones, from what people seem to be saying; good for gaming, and musically pretty balanced and pleasant to listen to unless you're a huge basshead. Some people allege that there is a "Sennheiser veil" which makes listening to tracks sound as though there's a curtain standing between you and the band. They also come with a cool headphone holder which clips on vertically or horizontally to a shelf or ledge. You can buy these for about $150.
The Beyerdynamic DT770's have a great deal of bass, which is their primary claim to fame (although the bass can obscure the mids and highs at times). If you love head-crunching bass, then these are what you're looking for. They're closed, so they can get hot, but they're comfortable thanks to their construction and you'll know where your enemies are. Speaking of construction, these headphones are built to survive World War III - the build quality is extremely solid and is not like to be an issue at all. Depending on which model of DT770 you're looking at, you may have to buy an amplifier. I'm honestly not sure about the street price for these.
The Beyerdynamic DT880's, built to last for many years like the DT770's, are extremely "neutral" semi-open headphones. Most headphones tend to emphasize certain things more than others, but the DT880's present sound with very minimal coloration. They are incredibly precise and this is why they are so popular with people who enjoy hearing stuff "as it was meant to be heard" (but this also means that low-quality sound will sound like crap). These headphones, unlike the DT770's, really don't have overwhelming bass or even above-average bass - bass is present, but it is precise and concise rather than big and overwhelming. The DT880's usually come very highly recommended for music listening and gaming as long as you don't mind its somewhat lesser bass. You can pick up a pair of the 2003-era DT880's for $150 - an absolute steal considering the new 2006 models (which I'm told sound more or less the same) sell for much more. If you bought these headphones, you would most definitely have to buy an amplifier - but I'm not sure what inexpensive amplifier you could pair with these to get good sound.
The Audio-Technica A900's are closed headphones which seem to be the favorite gaming headphone of many people because, according to them, it's very easy to pick out where your opponents are. The bass is very present and so are the highs, but the mids are somewhat recessed so these aren't as musically balanced as the HD 595's or DT880's. The isolation for these is said to be very good. The A900's aren't said to be very portable, however, so that's something to consider. You can pick up a set of these at Audiocubes for $169.
Keep in mind that I don't have personal experience with these headphones - that's just a composite of what people over at Head-Fi, who do have a great deal of experience, seem to say. If you'd like more information, surf on over to their site and ask some of their veterans. Given the choice between these, I've almost settled on the Beyer DT880's since they seem pretty win/win as long as you can afford them with a suitable amp, however cheap.