Here are the w7 system requirements. MS also provides an upgrade adviser software to check if your hardware is compatible.http://windows.microsoft.com/en-US/windows/downloads/upgrade-advisor
If you are compatible, you do not need to upgrade to each windows version, there aren't even upgrade versions like I think thently is thinking of. You just get w7 and install like any other pc, irrelevant of any other os version. Though I would assume such an old rig is not compatible.