|Richard's math is correct; he's just missing the "divide by two" after taking the square root :-)
Remember that good lenses are likely to far outlast your camera body; consider your lenses a long-term investment. If you feel you may ever move up to a camera with a larger sensor, consider that in the purchase decision. Also, remember that Canon's posted MTF graphs are theoretical, based on a software model of the lens design, and not on testing of an actual unit. As such, they represent what the lens *can* achieve, not necessarily what it *will* achieve. That said, I find them useful and a decent indicator of performance, though they really only tell you about sharpness and contrast.