This is news to me. I just got this email from Tyan's technical support:
Q: Why can't I get photoshop 5.x to work with my system?
A: After some research Photoshop 5.x does not support AMD cpus, only Intel. Photoshop 6.x has support for current AMD cpus.
Seems like this should have shown up a few lifetimes ago if it were significant (what about all those Content Creation benchmarks on Athlons we've seen for years?).