It supports 8 GB, however when I went from 2 GB to 8 GB, it did not make it any faster in Mac OS X programs. I suppose this is because the processor is the bottleneck. In windows 7, it made it a lot faster to have 8 GB RAM, even with the 32 bit edition.
Since 8 GB ram doesn't cost much more than 4 GB, you might as well go with the 8 GB upgrade. It's easy to install, and poses no more risk than a 4 GB install.