You should be looking more broadly. There are a great range of headphones which suit a variety of tastes and it's kind of unfair to simply look at a single pair as a possible choice.
For open headphones, have you considered the Grado SR-60, or a higher end Grado with amp? Or the HD580 with amp?
If you're truly going to a LAN party, in which there is a horrible amount of noise, rude comments, and stupidity, and you want to actually HEAR something without going DEAF, I suggest fully closed headphones that isolate several decibels minimum. The AKG K271S with amp works WONDERFULLY for me (although I don't currently own a pair, but they are great). Other phones you may want to try out include the Beyer DT250-80 (another great headphone for gaming, plus it's AWESOME with rock music and is comfy), DT770 (overwhelming bass and highs, great for first person shooters), and the Etymyotic headphones.
My friend purchased some ER-6 and they are perfect for the gaming he does: great detail, and 23 dBa of isolation.
Do the a900s isolate at all? If they don't, are you serious about buying them as a gaming headphone if you go to LAN parties? If you game by yourself and NEVER go to LANs, you are going to be fine with them of course.
Cheers,
Geek