So lets say mafia wars, would people be more motivated to play it if it was real cash or it will have the same motivational impact as virtual cash?

Let me build some context around the question: I am trying to build a weight lost game that ties in instant gratification with long term goals. There has been several studies that indicate if you give(or take away) someone money(like daily) that they will lose weight(http://www.foxnews.com/story/0,2933,464789,00.html?sPage=fnc/health/nutrition). So, what am trying to assess is whether it matters if it is real or virtual.