How interchangable are 700c and 27" rimmed wheels? I've heard you need new tires, and it would depend on the clearance of the frame, but would it be noticable to run, say, a 27" on the back and a 700c on the front?

Ideally, I wouldn't want a riding poistion like this http://www.wattagetraining.com/ttSetup/images/bike.jpg as it is supposed to be a winter training bike i'm building up

Asking out of desparation really, as I can't get hold of a 700c flip-flop rear wheel on my cash-strapped budget, but found a 27"