Close, but not quite perfect. You are correct that a magnitude 8.0 earthquake is ten times more powerful than a magnitude 7.0. However, a magnitude 7.9 earthquake is (approximately) 7.94 times more powerful than a magnitude 7.0, not 9 times more powerful.

To convert a (logarithmic) Richter magnitude number to a (dimensionless) linear scale for comparative purposes, use the formula:
10^Mag

So:
10^7.9 ÷ 10^7.0 = 7.94

Or (for the less mathematically challenged):
10^(7.9 - 7.0) = 7.94

P.S.: You can do similar comparisons with decibels (you have to convert to bels, actually) for sound intensity.

P.P.S: Sorry for scaring some of you with the crazy math stuff!