So far when you change from ANY Windows OS from 32 to 64 bit it's never an upgrade but a clean install.

You are likely to find that this system will run Windows 7 64 bit but be sure to look for all the drivers for Sound, Video since sometimes Microsoft doesn't get them for us.

Can I share my big secret to such upgrades without the trauma? Ready? I remove the old hard drive and carefully put that in a safe antistatic bag far away from the upgrade zone. Slip in a new blank HDD and try to install Windows 7.
Bob