All around the K5 would be a large step up in a number of areas. However, for sports - I believe that a K5II would be better, especially in the autofocusing department. Pentax made an improvement in terms of the ability to autofocus in much darker situations. The K5II has just been discontinued, however the K5IIs is still available. You can still find both if you look. The difference between the K5II and K5IIs is the lack of an anti-ailiasing filter.
The K3 is an improvement over the K5II/IIs in terms of autofocus in the tracking of moving objects. When you apply that to sports, I would think that the K3 would be the best option.