Across from Istanbul modern is Mimar Sinan Güzel Sanatlar Üniversitesi, which houses interesting exhibits. Right now they are showing a Dali collection that is quite nice. In the same area (across from the modern and on the Left of Mimar Sinan) there is a steep hill- if you follow it up, you'll end up in Tophane, which has a bunch of great independant art galleries.
Also, past the Galatasaray Lycee on Istiklal Cd, there are a bunch of great art galleries maintained by the banks. They, for some reason, really favor super avant-garde stuff, and should not be missed . Also, they are free.
