A monitor that is 15 years old is likely to be one of those fat CRTs right.
You probably could salvage the case.
Not sure this will help but i have a 12 yr old compaq computer (compaq deskpro), when i tried putting a asrock AM2 motherboard in, the stand offs were alittle higher than newer boards so the audio jacks wouldn't fit in the I/O plate...
I'd check to make sure the case front panel audio connector (if it has those cables) is HD audio and not AC'97. You lose automatic jack sensing if it's not. It's a minor inconvenience. However, is the case after market or OEM? If it's OEM there's a chance there's a proprietary power,reset,hd led, status led, connector being used which means you won't be able to wire it to your new board without cutting and redoing the connectors.
look at the connector of the monitor that plugs into the pc. if it has 2 rows of pins - its junk. Throw it out.
If it has 3 rows of pins then it will work in a new system as long as the new graphics card has a vga connector.
Anything else from the 15yr old system isn't worth salvaging unless its been upgraded since then. If it has a large hard drive or a dvd drive then you maybe able to use them if your new system has an IDE port on the motherbd.
The case may or may not work with a new motherbd as many prebuilt systems (HP, Compaq, Dell...) did not use standard layouts back then so a new motherbd might not even fit.