1, yes, all duel motherboards can support only one processor.
2, try the learn windows in 24 hours boks, they are quite good, and comprehensive, they cover quite a lot of stuff and are easy to digest...
though personally I'd recomend just asking simple questions here, (and other places), if you have only one server then there will be a lot of stuff in the books (specificaly Active Directory stuff) that you wno't realy need.
3, well that depends on whether you buy windows courses or not, and whther you want to buy a windows server, it also depends on what you want to use it for.
simple file server / FTP server / Web server / mail server??? not affraid to go in at the deep end running and get faced with a steep learning curve, want free software? then linux is the way to go...
want to buy windows, want a website with ASP pages, a Microsoft SQL database server? and exchange mail server? want a much easier ride than you'd get with linux?
then windosw is the answer...
as for the case?
where will this server go?
if you were thinking about having it put ino a datacentre or somthign for a massivly high speed connection, or if you wanted to become a web host, then you can't go wrong with rack cases.
but if you just want it to go under your desk, or in a corner of your house, even a server room, (on a shelf instead of a rack), then a reguar desktop case should do...
either full tower, (if you want loads of drives).
or midi tower, or even mini tower,
(full tower has 5 (or more) CD drive bays)
(midi tower has four CD bays).
(mini tower usually has two cd bays, or even only one)
obviously, the case bears no effect on performance unless there are problems venting hot air, in which case thermal protection can cut in throttling processors.