i think it is going to depend on what mobo you are using and the startup shutdown controller to power it. and any power accessories being used. in your case, i think it would be no computer accessories since its a race car.
what are the details of your car? and what car? mods? ive got a car im building up (finishing up) on. 87 fiero gt w/ motor swap that will get a carputer when i get tired of the monsoon deck i put in it. that will be a while.
is it OBD I or OBD II? i think either of those will need very little power to operate for data acquisition. 5V 2A at the most. if using one usb port off your desktop, then there shouldnt be any power problems.
what program are you using? im using the moates.net APU1 for data logging and tuning and using tunerpro rt.