Back in the day, huh? Gee, I wish I still had my 386dx40 with 4 megs of RAM.
. . .
wait, no I don't.
Sadly, a 400mhz Athlon64 will still outperform just about every CPU that was released at 400 mhz stock, due to IPC and other improvements in subsystems throughout your PC. It would kick the crap out of some k62-400, P2-400, or overclocked Celeron 300a.