The D7000 will not be overkill for you in my book. I ,too, am an amateur and I own both the D7000 and the D5100. The D7000 is easily my favorite although the D5100 is certainly a good camera. I just prefer the functions of the D7000 over those of the D5100 although the articulating LCD is the cat's pajamas on the D5100. I use it for macro work because I'm to old to be laying on my belly trying to photograph a bug in the grass. Since you are already comfortable with a D90, I think you will love the D7000. Just wait until you try the autofocus system of the D7000 on birds in flight. You will be impressed.
Oh yes, I forgot to tell you, I have not had any focus problems with my D7000 whatsoever.