I now have my system up and running on a 750W inverter. Here are the specs

120W ATX PSU
ASUS A7V400-MX motherboard
Athlon XP 1800 CPU
256MB Ram
Onboard Sound and Video
PCI Wireless
15" LCD Monitor

Now I'm trying to decide if I want to stick with the inverter or go with a DC DC psu. I really dont mind shutting it down and powering it back up. I'm usually in my car for extended periods of time.