I know you weren't looking for advice outside your request, but if you are in town March 29-April 1 you might want to check out the Association of International Photography Art Dealers show that weekend. http://www.aipad.com/photoshow/new-york/. Never been myself but might make it this time. It seems a great opportunity for viewing photography, about the same price as the Metropolitan or Modern but tons more photography to see.
Hope you enjoy your visit! I live about 2 hours from NYC but don't get there that often, and wish I could for an extended stay.
((The AIPAD Photography Show New York, one of the most important international photography art events, will be presented by The Association of International Photography Art Dealers (AIPAD), March 29 – April 1, 2012.
More than 75 of the world's leading photography art galleries will present a wide range of museum-quality work, including contemporary, modern and nineteenth-century photographs, as well as photo-based art, video, and new media, at the historic Park Avenue Armory in New York City's Upper East Side.))
Dont try the goodwill stores in the city. They are quite the disappointment for camera gear. I have a feeling they shuttle off their gear to the online site they have.
If you will visit 42nd St, which you probably will. The main Nypl branch has a nice free show going on called photography and ruin, show casing various prints revolving around the theme of decay and the passage of time.
Across the street is also the ICP which is doing a cool show on Weegee and also had a small section downstairs dedicated to the newly printed book on magnum contact sheet prints. $12 is admission.
I had the chance to visit both earlier this week as an errand brought me to the area and I had 2 hrs to spare.