I think for gaming, the AD900x would be a good headphone - mainly due to its soundstage when it comes to width. It's been awhile since I've had the AD900x but have been playing in the AD2000x and AD2000 territory. However, I found that many others also felt that the "x" series didn't really equate to an improvement. When it comes to the AD2000 vs. the AD2000x, the AD2000x seems to be missing the mid-range punch that the AD2000 was noted for. IMO, I'd expect a lot more from a $999 (list price) headphone like the AD2000x. Street prices and used are quite a bit lower.
With what I've stated above, it still wouldn't push me from the ATH-AD900x to the ATH-AD900. The previous version (non "x"), I found that the pads would easily bottom out and get compressed over time. This would mean that one's ears would come close to, or even actually touch the driver. The newer pads on the "x" series have addressed this issue. If money were in abundance for me, I would love to hear what the ATH-AD900 would sound like with the newer pads from the "x" series.
Finally, getting back to the ATH-AD900x. While I haven't heard the ATH-AD900x at the same time as the ATH-AD700, from what I remember of the AD700, it was a bit better. Some might say a bit less bass. Others might say a bit more of a narrow soundstage. I'd hate to tell anyone that the ATH-AD900x should be passed over at the price of $120. In that case, it might be a safe buy. If you don't like what it features, I feel that you'll easily get your money back if you had to resell it.