You may want to try 26 inch wheels rather than 650's.
There isn't a whole lot of difference between the 650's and the 26's. However, 26 inch tires seem to be a bit easier to come by.
We spec'd 26's on my wife's bike and it's made the bike more comfortable for her.
Here are the differences you'll feel going from a road to mountain bike:
1) The road bike will be lighter and will have tighter frame geometry. This means it'll be faster and feel more responsive.
2) The tires will be a little more flat prone.
Other than that, you will be much happier spending the extra scratch on a bike that fits you properly.
re-cur-sion ri'-ker-shen n: see recursion