uhm, simply, yes, it is possible to just take out a processor and put in another, of the same architecture. for example, you have a fully running p4 2.0 socket 478, you buy a p4 2.5 socket 478, you can just take the 2.0 out and put the 2.5 in.
as for dual, uhh, what do you want to know. they work pretty much the same as single processor machine. i dont recommend you get one because most thing that you will do wont even be able to use both processes and itll be a waste of money.
if you really want one, i recommend
supermicro . they make very very nice dual boards.
i just got just
this one and its so damn sexy.
. That ones prolly out of my league.