Can I upgrade from Pentium IV to Duo core with the same cabinet?

its very possible, but you have to do your home work. if you have a dell or something like that your job may be more difficult. If you have a no name machine or a home built unit you are better off. there are two major types of mother boards, ATX and micro ATX. You can to a search on them and find the specifics. open up your case and find out which you have and then shop for a mobo that will fit in your case. If you have a proprietary machine you will probably end up buying a case, no worries tho because a case can be had for as low as $30 if you shop.

If the motherboard will fit the cabinet, and the power connectors fit the motherboard, there should be no need to change out the box. So far, these things have been fairly standardized.

If in doubt still, it's best to check with the manufacturer of the new board. They would have the definitive answer to your question.

If you can get a new motherboard to fit into the same case (case not cabinet) then yes you can.

If you have a machine with a proprietary motherboard you may not be able to get a suitable board to fit.

cabinet? you mean the case? no, most motherboards will fit a standard tower, but check before you buy