A couple points then some pictures. I think if you have grown out of the 3100 you need to go higher than a 5100, the D90 would at least give you more control over your shots on the fly than the 5100 if you already know what your doing.
As for the original question about the lens purchase. I have rented the super bad ass 80-200 f 2.8 VRII telephoto from Nikon, in short for taking action shots, quick, quiet autofocus and amazing build quality and glass and you are serious about shooting, save up the dough for it.($2k+) The VRI is still very nice and used can be had cheaper. ($1200ish) or you could buy the Tamron which has a little slower and louder AF but is again cheaper.
Although for reference the shots below were taken with the 55-300 f/4.5-5.6 lens.
Also these shots have only been corrected in Lightroom, No Bokeh was added in Post.
http://i224.photobucket.com/albums/dd58/reitzmj/DSC_0075-1.jpg
http://i224.photobucket.com/albums/dd58/reitzmj/DSC_0493-1.jpg
http://i224.photobucket.com/albums/dd58/reitzmj/DSC_0584.jpg