It depends on what you are looking for, but you are most likely going to want to head to Chelsea which is where all the major galleries are now.
Also when are you going - a lot of galleries close for all or part of August.
The major galleries are Mary Boone, Andrea Rosen, Cheim & Read, Gagosian, Matthew Marks and a bunch more plus there are lots of smaller galleries. All you have to go is go to 10th avenue and from like 20th-26th streets it is door to door galleries.
There's also the Dia Art Center right in the same neighborhood.
I like to check the Douglas Kelley Show site for gallery openings. It's always fun when there are a bunch of openings on the same night because you can hop from gallery to gallery and the streets are crowded with people.
You can get a list of galleries and addresses here.
Also if dot is around she'll have advice since she used to work in the gallery world.
Oh no! lol, I had no idea, and I'm going right in the middle of August. I guess I'll have an excuse to visit again soon, and this time I'll have more time for museums and shopping