For sheer MHz, I went from a Celeron 575 to a Celeron 952.
For performance difference, I went from a Cyrix PR166+ to a K6-2 400.
That was a pretty big jump.
Basically my entire CPU history:
Intel Pentium 100
Cyrix PR166+ @ 200+
AMD K6-2 350 @ 400
AMD K6-2 450 @ 504
Intel Celeron 400 @ 575
Intel Celeron 566 @ 952
Intel Pentium III 700 @ 966 (1001 if I use my noisy ass PEP66/Delta38 combo)
Viper GTS