It is always recommend that you get Windows 7 pre installed with your new PC. This is the best option to get Windows 7 for you. The other alternative option, buy new hardware and purchase a retail version of Windows 7 and then perform a clean install of Windows 7 on your existing PC.
Its not recommend to install Windows 7 over Windows Vista because installing a Windows 7 on top of Windows Vista may cause incompatibility problems which will not be fixed easily.
When you buy a new computer with Windows 7 pre installed, it will ensure that that the components in the System will have been selected for their compatibility and will have the latest drivers.