just a quick and kind of pointless side not...the LGA775s aren't in reference to pins in the same sense the other ones are....LGA is a 'Land Grid Array' meaning the pins are in the motherboard, and the CPU has little 'spots' on it that the pins in the motherboard make contact with...really weird, but yeah..was just a little side fact.
when AMD's first came out they had socket 754, socket 939, and 940, no one knew what to choose from at first...but socket 754 didn't have dual channel capabilities, and beyond that they held some of the lower processors, like it was mainly for 2800+, 3000+'s like that...socket 939 was meant basically for the desktop environment, included dual channel RAM capabilities, and housed the higher up CPUs, like 3800+, 4000+, etc..etc..
Socket 940 was primarily for servers and the FX chip was originally on this one, but then later became available for the Socket 939 series....socket 940 is meant for opterons (IE Server CPUs) and requires registered RAM with ECC
It was in this last couple of years where different sockets were really confusing, but for the most part like fghtin said, the trend has been the bigger number, the more pins, the better the socket and it's capabilities