In general I would always recommend a clean install rather than an upgrade. Obviously make sure you have all your driver installation files ready for Windows 7, and any software installation files you need. Get a copy of your data. Boot from the Windows 7 DVD and run a fresh install.
Be aware when moving to a 64-bit version of Windows that whilst some 32-bit software will work fine, some will be slow to perform and some will not work at all.