server motherboard form factor

Are there currently any ATX pc motherboards capable of running multiple CPU's simultaniously?
I have seen server boards that can run off of 2 or even 4 CPU's, but i wanted to know if there where any standard pc ATX form factor boards that will do the same thing. Also, what software/OS cosiderations would there be in such a setup? if anyone knows the answers please say. Thanks!
Tyan motherboards (Tyan Thunder and Tyan Tiger)are a well known 940 socket mobo that do this...very expensive though. Asus makes a few I know...like the K8N-DL model (way less price than the Tyan).
AMD Opterons are a common dual CPU setup...the 940 socket, not the 939's.
As stated above, you will need multiproc OS like Linux or NT...basically a server OS.
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