Vista 32 bit only recognizes 3GB of RAM, unless you enable PAE. PAE has some overhead associated with it, so it's really not optimal for best performance. Also, PAE will enable if you install Vista on a machine that has 4GB, but it will not automatically enable if you upgrade the RAM after installation.
Installing Vista SP1 enabled it for me, or alternatively you can following the instructions here.
http://www.bleepingcomputer.com/forums/topic93761.html
Heed the warnings about the sound card, I have an X-Fi, and I was getting BSOD within 2 minutes of starting the system with that card in there.