You can use a server motherboard, it's the OS that determines whether
it's a server or not Main issue is whether it will fit the case
as they can be proprietary, same with the power supply - not usually a
standard ATX.

If the old PC does not boot up and no power. The power supply may be the
issue. Just get a new one and pop it in.

Thanks. I'll try and see what happens.

ANyone know what voltages should be present on drive plugs from
the power supply?

Many power supplies won't provide any power to any of the plugs without some
load on the power supply. 12v, and 5v (both + and -) should be present.

There's a colour code for these:

Yellow = +12v
Red = +5v
Orange = +3.3v
Black = Ground

Other wires are:
Green, Grey, Purple and sometimes white.

However, voltage is only part of the story. These voltages need to be
stable over a wide range of current draw and also have minimal ripple
on them. It's possible for a PSU to give all the correct voltages but
fail to operate a motherboard properly, or cause instability due to
poor regulation under load.

If in doubt the acid test is a replacement PSU.

FWIW - on opening up many marginal or failed PSU's I've found the old
'bad capacitor syndrome' and in one spectacular case one of the coils
had failed insulation and had burned black ( customer had reported a
flash and bang from the back of the PC !! )

If it works. It works. Just make sure your are grounded before handing any
circuits. The electrostatic shock from your body can cause walking wounded chips
on the board. Please post a pic of your creation. Love to see what you come up
with.

I put a dual-Opteron server motherboard in an old Gateway 2000 full tower case,
but I used the reset button as a power button. Depending on the server
motherboard you may have less PCI slots to work with, but more PCI-X slots. You
may have to use a PCI video card. If you need to you can run sound off a USB
sound card.