It has the same limitations as XP does, due to the whole remapping of PCI address space. For me, I only had access to 2.9GB in Vista32.
I ended up biting the bullet this weekend and moved up to Vista x64. Went very smooth; no compatibility problems with anything (yet?). 64 bit LAME encoder runs 40% faster! Nice to have 4GB finally. I was suffering for a while under XP with just two 2GB of RAM. While building a new machine last month with 4GB, that's when I learned about the whole 4GB 'issue' under XPPro and Vista32.