The number associated with an ND filter is actually the denominator (bottom) of a fraction.
So an ND2 filter should be thought of as 1/2 the amount of light being allowed through the filter. For example, setting the lens at f/2.8, and using an ND2 filter would make that an f/4 situation for a total of 1 stop difference.
ND4 filter is allowing 1/4 the light (which is half of ND2) thus a 2 stop difference.
Continuing, ND8 is 1/8 and three stops and, although I've never seen them, an ND16 is half as much light as ND8 so would be four stops less light.
The decimal numbers you mention (0.6, 0.9) are another way to quantify the density of the ND filter. It basically relates to some esoteric optical quality that engineers probably like to toss around while having coffee.
I would highly suggest the best quality GLASS filters you can afford. Cheaper (especially plastic) filters will tend to add nasty color effects. Although technically color casts can be corrected in post, cheap filters also can also reduce the quality of light meaning things like more chromatic aberation.
Lastly, don't worry about getting the highest ND number, I carry two filters around and stack them together, when needed, for combined affect. Which is more reason why quality filters matter as stacking simply magnifies imperfections too!