Also, just an FYI Vista supports 4 gigs of ram on 32 bit, and 128+ gigs on 64 bit and XP only supports 3 gigs, and 128 on 64 bit
As I said before I am actually running XP 32bit with 4Gb and it works fine. Sure, it doesn't seem to be able to address 100% of that last Gb, but it reserves about half a Gb up to 1Gb for other system resources. This is not different with Vista 32bit actually.
There you will also only have about 3.5Gb of ram that you can really use when you put in 4Gb.
64bit versions of Vista allow;
Windows Vista Home Basic - 8 GB of RAM
Windows Vista Home Premium - 16 GB of RAM
Windows Vista Business - 128 GB of RAM or more
Windows Vista Enterprise - 128 GB of RAM or more
Windows Vista Ultimate - 128 GB of RAM or more
The 32Bit versions only allow 4Gb,
Cheers