Section width is the overall width of the tire at its widest point, when mounted on a wheel of certain width (known as the measuring rim width). Actual section width of a given tire varies a bit with wheel width, but the "205" is a nominal figure based on an agreed-upon rim upon which the tire is mounted before taking actual measurements. As you can imagine, a "taller" tire bulges more than a low-profile tire, so there is more difference between section width and tread width in a 70-series tire than there is in a 45-series tire. In fact, you may have to go to a wider rim to fit the same section-width tire in a lower profile, since there just isn't as much room to flex in the sidewall.
I'm not sure how much the contact patch would change. It's pretty much true that for the most part, it's not the size of the patch but the shape that changes. Now, if you're asking about the difference between a 205/45-14 and a 205/45-17, I would guess no difference. But between a 205/70-14 and a 205/45-17 you would notice a significant difference in turn-in and other handling measures. Of course, finding a "performance" tire in 205/70 is pretty much impossible, so you're really talking about tires designed for completely different purposes in that case.
