L337,
to my recollection the 737 - 200 flies at about .72-.73, the 737-3/4/500 flies at .74-.75, and the -737-6/7/800 flies at .78-.79 - ish.
It depends on your fixed costs against fuel prices, if the cost index is high as it was in the late 90s, the -800 will cruise happily at .795. In my current Company, the -700 cruises at .78 with a much lower cost index because of higher fuel prices, but it does it at FL 400 and FL 410.