the dt990pro is a good set of headphones. afaik the pro is only available in 250ohm so you would definitely want something to power them.
the cheapest card which would do the job is a xonar dg for $30, however quality is really only as good as your onboard but it does provide virtual surround and a more powerfull headphone amp. of course you could hook up a cheap fiio amplifier up to your current onboard and accomplish something similar but in a more clunky way and with no virtual surround. a better choice would be the soundblaster z card for $80
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the ad700x, ad900x, hd558 and hd598 are all common gaming headphones worth looking into. the ad's have wide soundstages but poor bass. the 598 has slightly better bass but slightly smaller soundstage. the 558 has an even smaller soundstage but more bass than the 598. the 990 is going to be more bassy then any of them and is generally a good choice for people who want open headphones without sacrificing too much bass output.