Either way you need to clean install. Your options are Windows Vista 64 bit, Windows 7 64 bit or Windows 8 64 bit. You can use a Digital River 64 bit setup files with Service Pack 1 to make bootable media and clean install 64 bit Windows Vista Business.  I recommend Windows 7 Professional with Service Pack 1 64 bit but you will need to pay for a license (its probably the most expensive option). You can alternatively upgrade to Windows 8 Professional, the upgrade license is cheaper than that of Windows 7 but in my opinion an inferior product. I would recommend trying out either and deciding what you like the most and if you think its worth upgrading. Follow the instructions in A Clean Install of Windows: [URL] I advise carrying out Step 2 and 3 of A Clean Install of Windows Vista before experimenting. The guide tells you how to get trial versions of Windows 7 and 8 so you can evaluate them. With Vista 64 bit you have the product key and should be able to activate using the activation backup recovery program or phone activation to Microsoft.
I recently purchased a used Dell Precision T5400 with Windows Vista Business. I'm very happy with it, but I am planning on running a 64-bit program on the workstation. What are my upgrade options?
Upgrading Vista Business to 64-Bit on a used workstation