Although theoretically 29" and 700c are the same, there are several factors to consider, and you may run into compatibility issues depending on the specific rim/tire combo. Also make sure that the frame is wide enough, and that there will be enough clearance all around, as wider tires will increase the total diameter of your wheel more than skinny tires.

You may wish to read Sheldon Brown's article on tire sizing.
http://sheldonbrown.com/tire-sizing.html

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Two wheels good. Four wheels bad.