I agree with Mr. @cobito
@DanHDU As a former owner of an Athlon64 3000+ I can confirm that its natural environment is indeed Windows XP, where it moves like a fish in water.
If you're going to put Windows 7 on it, I would recommend Windows 7 Starter, as its limit is 2Gb of RAM, which is probably the limit of the motherboard you're using. It has fewer functions than the normal Windows 7, but it will run quite smoothly as it requires fewer resources.
Best regards!