Doesnt sound too bad. although most of us around here use Via Epia motherboards because they dont take much power and are VERY small, but make sure you keep your system nice and cool, that athlon 2500+ could get pretty hot in that car when its running on a nice hot summer day, so a few nice fans and a good heatsink should fix that. As for the hard drive, i have a western digital hard drive also and ive had no problems yet. If you hit a good bump youll know it, the hard drive will reset and that will be when you know to do something to support it.
what are going to use for power?