Pentium MMXs are much faster than the old Pentiums at the same MHz ,needless to say the PII's are much faster.
The PII is a tiny bit faster than the Celeron PPGA/Slot 1 at the same clock speed & FSB,the 100MHz FSB PII's are much quicker.
The PIII Coppermines are a bit faster than the PII/III non Coppermines & the 133MHz FSB versions faster still.The fastest being a Coppermine PIII on 133MHz FSB.
The Celeron FCPGA is much slower than the equivalent PIII due to FSB & a crippled cache (1/2 the bandwidth as well as size).Even the 100MHz FSB versions don't compare well. Tualatin Celerons are virtually PIII's (256k cache)on a 100MHz FSB & thus do much better ,they benefit significantly from overclocking the FSB though ;)
Tualatin PIIIs,especially with 512k cache are much faster than all of the above.
P4's do well only when coupled with DDR or RDRAM & at higher clock speeds.The Northwoods are best with 512k cache
AMD K6 family chips suck (at least 40% slower compared to PIIs),the K6-3,2+ & 3+ fair a little better.
Durons Spitfire (upto 950MHz) compare well to the Athlon Classics Athlon XPs are faster than the Thunderbird versions with the same RAM Duron Morgans (900MHz+)are also faster than the Tbirds at the same MHz! (in SETI anyway)
Athlon CPU's are similar to the PIII at equivalent MHz and using the same RAM. P4's need to be about 25% (this is an estimate) faster MHz to match an XP Athlon with DDR RAM
Important factors in crunching speed are memory timing (e.g. CAS-2), and bus speed (e.g. 66,100,133,166 MHz)& the latency of the chipset (lower is better).