One commenter confessed to being confused by conflicting information he’s read. Vista x64 does indeed use more memory than its 32-bit cousin, but in my tests on a dual-boot system I found the difference to be about 12-15 percent for the operating system and its essential services.
I have one test machine that’s ideally suited for this purposes, being equipped with 4GB of RAM, two hard disks, and separate installations of x86 and x64 Vista Ultimate running on separate drives. On this test platform, I found that the x64 machine consistently uses roughly 1.05GB of RAM after startup, while its x86 counterpart uses 935MB to run roughly the same number of processes. (These numbers are consistent on other systems as well.) On the x86 machine, the top 11 processes use 220MB of RAM, while the same group of processes at the top of the x64 list use 251MB.
Measuring memory use is tricky. For example, the x64 systems occasionally grabbed extra RAM from the available pool, presumably to use in building the page file and Superfetch cache. In all cases, though, memory usage eventually retreated to a baseline level and stayed there.