There are a number of interesting galleries in the Chelsea area, around 23rd to 25th street or so, way over on the westside. I'd also say a trip to NYC would not be complete without going to a museum or two, and the Museum of Modern Art has an excellent photography department.

Another great resource is a publication called Photograph, and can generally be found for free at just about any gallery. They list just about every photography show in the US, with a large NYC section, and the listings can also be found online so you can plan what to see before.

Have fun!