It should not matter that your Vista is an OEM version, you should still be able to upgrade from Vista Ultimate to Windows 7 Ultimate (if you are going from 64-bit to 64-bit). You can choose from doing an in-place upgrade (which will keep your files, programs, and settings intact) or a custom installation which will wipe your computer and allow you to start fresh. Either way, it's a good idea to back up anything you wish to save before upgrading. Visit http://windows.microsoft.com/en-us/windows7/help/upgrading-from-windows-vista-to-windows-7
for a guide to help you through the installation process.
Microsoft Windows Outreach