i used to have both Vista 32-bit and 64-bit during they were beta, but have not tried in the release versions since i will need to have another Vista's licence. personally, i think there is not too many complications to dual boot them if needed, and based on my past experience the dual booting is not a self-complicating system at all.
as for about answering the op's original question, this below Microsoft article may not explain well, however, this is more about 4 GB of RAM installed and Vista
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The system memory that is reported in the System Information dialog box in Windows Vista is less than you expect if 4 GB of RAM is installed
this below article talks about the set PAE forceenable. i personally don't recommend using this method, the Windows system may not stable due to problem of some device drivers.
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Feed 4 GB of RAM to 32-bit Windows Vista - No more memory limitations - Softpedia