Why why why!?!
To be honest I don't think either of these are the best choice.
Recommend you go with Audio Technica AD700's- they are easy to drive, great value, have a good soundstage and comfortable (important with hours of gaming).
AD900's are better in detail and have more bass but if you are going to use the phones purely for gaming I don't think you would notice much difference.
Search gaming community forums on either of these if you don't believe me, they both are popular and highly reccomended.
AT A900's have stronger bass but don't offer the same soundstage or detail and can often get hot to wear.
Spending the sort of money on 701's or 880's without decent amplification is like driving around in a ferrari with your tyres flat- it just won't perform to its maximum potential.
But I guess if you have your heart set on these two its a good thing value is percieved, and so too audio quality.. (which is evidence enough that I could be wrong as well!

)
Can't comment on the Ultrasone's.
p.s I thought DT880's were good on my Auzentech Prelude until I heard them on a decent amplifier. I now have AD900's for gaming and DT880's for music.
