ok, i'm foaming at the mouth at the thought of getting a pc up&running in my car. but i'm on a budget (a "wife&kids" budget, if you know what I mean

)
anyway, i've got a pc chips 748 motherboard lying around doing nothing, and so in order to keep costs down, i'm wondering if/how this will work as a basis for a car pc. it's got onboard video/sound/lan/etc, and it's currently got a slot-1 celeron (400 MHz?) CPU in it. The board's got both a slot-1 and a socket-370 on it, so I was thinking of upgrading to a P3 FCPGA CPU. The board's got both AT and ATX power connections on it, so I can use an ATX power supply (good thing), and the board's form factor is baby-AT.
So, I guess my biggest question is, how do I know how much of a power supply it will need? Is there any easy way to figure this out? (I'm a software developer, so I'm not stupid, but I'm not a genius when it comes to electricity and stuff like that)
I guess the second question is, assuming I upgrade this to a P3 CPU, I imagine it'll be decent enough to handle XP and MP3 & GPS software, right?
Thanks for any help.