AMD X2's and 165 to 180 Opterons work on all socket 939 motherboards (Opterons not officialy supported, but they work.) (Some motherboards may need a BIOS flash for the second core to be recognised and/or by active.
Socket 940 from AMD is the server CPU's, and you can see which boards can support two CPU's, as the have 2 or more slots for them, I think the 840 is dual core.
Intel have their dual core ones which are on socket 775, and will work with all 775 motherboards.
The Xeon's are their server CPU's and you can see on the motherboard if you can put 2 of them in.
Cisco CCNA, Comptia A+, 1/3 through CCNP