Second only to getting an SSD in my 2006-vintage laptop the AMD 5x86 was the best upgrade I ever put in a computer, dollars for performance. It made my 486SX-25 run like a 75Mhz+ Pentium. Linux kernel builds flew after that. I also fondly remember seeing little bits of "Second Reality" run better or show me things I'd never seen before (more of the sword extending further from the pool, as an example).