Memory shows in system incorrect on new Vostro.

Hi. I just bought a Dell Vostro with Vista.  The RAM is showing as 3GB in System details, but the sales-person told me it would be 4GB (2x2).   Is there any way that it could have 4GB and show only 3 in the System folder?   Customer Service told me it's something to do with Vista that it only shows as 3GB, but a friend tells me it doesn't sound like a reasonable explanation. 

Could I open the memory card panel underneath and have a look?  Will it be written there somewhere.

Thanks very much in advance!

Dave