You will need to do a clean install, but you can use Easy Transfer Wizard
according to some posts. Otherwise I would dual boot until you are
satisfied you have everything moved over.
As John says, use the Windows Easy Transfer tool to save your files and
settings from Vista x86, clean install Vista x64, and then use WET again to
reinstall your files and settings. Then reinstall your apps. Since you are
moving from x86 to x64 on the same machine you will not be able to use the
WET Companion to move the apps.