f1.4 to f2 is a full stop, so to 1.7 it is a half stop to 1.8 2/3 of a stop. for sports all those lenses are pretty short for getting in close.

full f stop jumps are

F1, F1.4, F2, F2.8, F4, F5.6, F8, F11, F16, F22, F32, F45, f64, f90, f128....

the wiki on it is quite easy to understand actually

Quote: In photography, stops are also a unit used to quantify ratios of light or exposure, with one stop meaning a factor of two, or one-half. The one-stop unit is also known as the EV (exposure value) unit. On a camera, the f-number is usually adjusted in discrete steps, known as f-stops. Each "stop" is marked with its corresponding f-number, and represents a halving of the light intensity from the previous stop. This corresponds to a decrease of the pupil and aperture diameters by a factor of \scriptstyle \sqrt{2} or about 1.414, and hence a halving of the area of the pupil.

Modern lenses use a standard f-stop scale, which is an approximately geometric sequence of numbers that corresponds to the sequence of the powers of the square root of 2: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45, f/64, f/90, f/128, etc. The values of the ratios are rounded off to these particular conventional numbers, to make them easier to remember and write down. The sequence above can be obtained as following: f/1 = \frac{f/1}{(\sqrt{2})^0} , f/1.4 = \frac{f/1}{(\sqrt{2})^1} ,f/2 = \frac{f/1}{(\sqrt{2})^2} , f/2.8 = \frac{f/1}{(\sqrt{2})^3} ...

Also you have to take into account how narrow DOF gets when you use fast lenses at 1.4-1.8 in close nailing focus in action shooting will be tough. most sports shooters use a constant aperture Zoom say a 70-200 f2.8 which would be a little over a stop slower than an FA77 but let you frame better (and will mean moving from iso 800 to iso 1600 for instance (or 1600-300, 3200-6400..... hence the value of a K5

)

EDIT: I see you shoot with a K7 so i understand why you might want the extra bump at the lens as you already have a 2.8 70mm. your long lens though is quite slow. a Tamron 70-200 2.8 and shoot at 1600iso on the 7 with some judicial noise reduction may result in more keepers than an FA77 @1.8 (lots of other reasons to want an FA77 though, just not for sports IMO)