Once again Bruce has given great information.
 
One thing I would suggest: Your vet might want to do an Intravenous
injection of dye and take an x-ray afterwards to see if the kidney without
the cyst has normal blood flow.  I have seen a couple of cases where the
blood work before sx was normal the cystic kidney was removed and the pet
went immediately into renal failure after surgery.  Because even though
the kidney had a big cyst on it: that was the normally functioning kidney,
the other one did not work well at all.  This test is often called an IVP.
 
Just some food for thought.  but besides Bruce is right, usually these
cysts are nothing to worry about.  I see them all the time.
 
Benjamin A. Otten, DVM
Avian & Exotic Pet Medicine
The Animal Medical Center
[Posted in FML issue 3359]