I think if you read any of the in-depth reviews around the web, for the majority of serious gamers, the thing that makes the biggest difference in respect of sound-staging is not the headphones but using a mix-amp. Personally I loved both the PC350 and 360 combined with the Astro Mixamp, but as I have said it is the mixamp that provides the edge not the headphones.
Headphone choice is highly personal and the OP should consider comfort as well as performance. If you are going to be wearing your headphones for hours at a time, there is not point in having the worlds best headphones if they are as uncomfortable as hell and you dont enjoy wearing them. Closed backed earphones generally provide a lower frequency bass response, but many gamers complain about their ears getting too hot and uncomfortable. Open backed earphones suffer less, but allow ambient sound into them. Think about your head size, some earphones do not suit people with particularly small or large heads. Closed backed headphones are great for total immersion, not so great if you have small children that you may want to hear if you are gaming in the evenings.
The reality is you need to try headphones on, for at least an hour or two.
As for frequency response, well there is a reason that gaming headphones have a generally narrower frequency response than some of the better audiophile headphones. Game developers generally develop the audio of their games in the 20hz - 20Khz frequencies, because that is the approximate range of human hearing. Musical instruments (non electronic) by their very nature often produce sounds beyond these frequencies that cannot be heard directly by the human ear but can often be percieved by the brain almost subliminally. Hence a good quality audio headphone will often be quoted as having a frequency response beyond the 20hz - 20khz that cheaper headphones do not as these additional frequencies are what add the extra colour or dimension to especially orchestral music. As Shamrock says, how the headphones perform across the entire frequency is what gives each headphone its individual tonal chracteristic, but lack of ability to perform beyond a frequency range will limit a headphone as to how it reproduces not sound.
