I've been negotiating with that problem as well, except I've also had to handle having large ears and seeing my options and choices diminish dramatically as a result.
In the end I've gone with the AKG K550—they'll arrive later this week. They have large earcups, effective isolation, wide sound stage, and detailed precision in the width and depth of the field. I wanted an all-round set of headphones for everything, but eventually I had to sacrifice on portability and go with the K550's. The Philips L2 were also a viable option as apparently they possessed a wide and deep sound stage yet isolated noise better than the X2 or other open-ear equivalents.
If you have small or 'regular' sized ears, the world is your oyster. Basically anything will be effective, especially ones with a renowned sound stage and neutrality to the signature. The NAD VISO HP50's and PSB M4U1's are an example of this. You mentioned the Momentums... they're supposedly very good headphones for a variety of styles, but I don't know how functional they would be as a gaming headset. I can't imagine the small earcups would be comfortable for lengthy gaming sessions.
You also should consider the Audio Technica ATH-A700X and A900X models.