Is there a big difference in 1.9 gHz and 2.0 gHz processing speeds?

yes, 5% :).
The processor is no longer the real indicator of speed, although the computer companies would love you to think so. What really slows down most computers is not enough RAM. Make sure a new computer you are buying has 3gb of ram and you should be fine. Extras like dual/quad core are good for high end users, but not really necessary for standard users.