there is no "typical" power draw, some PCs use less than 50watts, others use over 500watts.
when determining the power requirements of a PC, there is no difference between a standard home (110v/220v AC) ATX PSU and a car (12v DC) ATX PSU. they both output the same voltages (+12v -12v +5v +5vSB +3.3v). whether using a home PSU or car PSU the same basic rule still applies; the PSU's total output wattage is a useless rating when determining the power needs of your system. you need to determine the current draw on the individual voltage rails (mainly the +12v and +5v rails).
this FAQ has some good info as well as a link to a CPU electrical spec chart where you can find out your processor's power draw.
you can find the amperage ratings for each DC-DC PSU in the mp3car store under the "technical specs" tab (under the product picture).