Not sure if you can take many to be honest, check that it's an ITX motherboard, because a P3 will probably be AT/ATX or mATX which won't fit in a Voom2, you could always custom build a case and get an intelligent PSU, probably a M2 or M3-ATX would do nicely. You can take your HDD, Ram, Sound card, VGA card as well, but they'll all need a custom case as well
I reckon, go to your local hardware store, get either perpsex or aluminuim and start drafting up case designs i'm doing mine now, in full perspex double DIN and it's running great, just working on the final DIN cover, I'll post a full write up eventually, when i can be bothered
i think darquepervert above means "use the whole thing" just as he wrote it:
use the original case etc, get a M2-ATX or similar power supply, remove the old PSU or even use an inverter(although I would rather use a standalone instead of going 12v - 120/240v - 12v again, no point in doing that to be honest.
Then find a place in the boot to mount the box, run remote USB and VGA cables to the front and maybe a USB CD-rom or DVD-rom and you'd have yourself a nice base unit