So you've got the 32bit version of Vista, & 4GB of RAM. All is not lost! The OCModShop guys show you how you can get Vista to "see" all (or nearly all) of your 4GB.

"Unfortunately, users of the 32-bit flavors of Vista have to run through some hoops to see all of the new RAM they've upgraded to. Even then, they may not see all of the RAM available because of various hardware issues."