Given it's 10 year old hardware, it's probably not 64bit capable anyway.
One thing to keep in mind. If you want 64 bit Win8, you need to make sure you do the upgrade assistant on a PC running it. I installed an old copy of Vista on my desktop with a valid license, but had to do the upgrade on my laptop with Windows 8 Enterprise Eval so it downloaded the 64 bit version.
Given it's 10 year old hardware, it's probably not 64bit capable anyway.