[Tarquin]

I know some gamers with pretty awesome audio setups. Mine is about $300-$400, and works really well for me, and while you could spend a lot more or a lot less, I think a reasonable investment in audio equipment should be about at least this much (assuming you have the cash) because it will pay dividends for years to come, while a gaming rig needs to be replaced every 3-5 years.

The audio section of the Hardocp forums is really awesome, and a good place to check out to see what kind of audio equipment gamers like.