[H] P4 explained

[Winterlight]

there is a half a dozen variations on the P4 now, Is there a site that does 
a good job of explaining all these.

For example... the latest ones use the LGA775 socket but why are some 
advertised at P4 550 3.4ghz 800Mhz FSB 2MB LGA775,  ...and then another 
one  at P4 650 ? What does te 550 and 650 refer to?