How can I find out the maximum magnification of my telescope?
The magnification is given by (focal length of telescope)/(focal length of eyepiece).  The spec I saw said the telescope focal length is 800mm, and the maximum magnification is 160x, so to get that would need a 5mm eyepiece.

Barlow lenses have the effect of increasing the telescope focal length: the 2x barlow would therefore give an effective focal length of 1600mm, and using the 9.7 mm eyepiece would give magnification of 160 (approx).  However be aware that the occasions when 160 magnification gives really clear views will be quite rare, as it is at the limit of what an 80mm scope can achieve.  The rule of thumb for maximum useful magnification for a good scope is 50x the diameter of the objective lens (in inches).