I am suffering from a severe case of lens lust. I can pick up a used Nikon 600 AF ED-IF F/4 tele for $ 2900.00 US Or a new Sigma 300-800 F/5.6 zoom tele for $ 3,000.00 Whats a guy to do? I am looking for the extra reach for birding. I currently have a Nikon afs 200-400vr and a Nikon 500mm F/4 AFS II; but I want more ........ More reach. Somebody please help me! Is $ 3000 for a used 600 thats probably 10 years old but in mint condition to much to pay? Any recommendations would be welcomed.