Seems it's between 3.2 and 3.8, it's 3.5GB that is supported
"...Due to these limitations, 32-bit Windows Vista will only be able to address approximately 3.5 GB of RAM. However, there is a way around this. One that will enable you to enjoy the full extent of 4 GB of memory. Still, you should be aware of the fact that in the vicinity of 4 GB of memory, 32-bit Windows Vista will not manage RAM efficiently. "
Windows Vista 64bit support 128GB of ram, so this will take some years/decades when this is in a mid-range system.
