Tsuyoshi -
Are all the walls perimeter wall (apart from the party wall at the stair)? This might tell us more. Metal studs are okay - if the gauge is high enough. i'm just prejudiced because they are so rickety. I greatly prefer wood. It's going to make your life much easier later on to use SOME sort of backing. This is not meant to be any sort of guggenheim-grade (which isn't that fancy itself) - just something serviceable and straight.

5/8" would be the minimum you would want to use for any sort of wall that is going to be scrutinized in any way. It's much stiffer than 1/2" and therefore straighter - and just looks better. Do you have a plan from the architect in pdf or other format? Can you post the rendering here? If it's too big - just PM me and I'll send you my e-mail. I'll do my best to give you good advice.