Originally Posted by Aluminum
2xx required, socket 940.
1xx are socket 939, and do not have enough HT links for SMP, besides their own cores.
They are usually just Athlon 64s with 1mb L2 per core. The 165 was somewhat popular with overclockers when it was basically the same price as a 3800 X2 with double the L2 and reached just as high on clockspeeds. Unfortunately, the price cuts did not touch them at all and there are no AM2 versions. There is almost no reason to buy one today :(
Although the AM2 4000 and 4400 also have 1mb per core, they discontinued them almost immediately and did not have the price cuts. I did see a supposed lower thermal AM2 4000 on one site currently priced between the 3800 and 4200, but I'm not sure if its a misprint.
Older Opteron 1xx were originally s940 actually. When AMD moved to fab 90nm from 130nm, they did not make 1xx series Opterons in 90nm (s940), instead moving the single socket server/workstation platform to s939 in order to use cheaper non-registered DDR1.
The important thing here is paying attention to the socket. SMP requires two things, s940 and either 2xx or 8xx series chips.
The old s940 1xx series Opterons were single socket server chips as well.