DC-DC ATX power supply

No you need to check the input rating  The 300 watts is the output only   that number does not include the fan or the efficeincy losses of the supply.  if the input rateing is in amps multiply the volts X amps  ie 120 X 1.2 amps= 144 watts @ 120vac  so get an inverter that could do 150 to 200 watts.

coo thanks....also let's say i turn off the engine to chill...i know the power stays on but if i wanna start the car...would it reboot the comp...if so where or what would i do to make that not happen...case it would be a hassle turning it on and off when u make a quick stop

The point about the system rebooting is exactly why I would recommend using an acual car pc power supply for this application. It will actually control the motherboard's soft power functions based on voltage from a remote turn on lead. Very cool stuff. Using an iverter, while it will certainly function is too me kinda like a hack job. But that's just me. And I havn't done THAT much research, there may be a good way to control the computers powere with the inverter/standard power supply method. Sorry if that's long winded, but I hope it helps. If you haven't already, read the manual for one of those car pc powere supplys off of mp3car.com. I did and it was quite enlightening :-)

Ok, it just so happened that after posting I happened on another thread that listed a product that could be used to control a system's power. www.mini-box.com it's under power solutions>ITPS (if you hadn't previously read that post) that would make a somewhat more polished setup, although it doesn't have as many shutdown options as some of the 12V-12V power suplies. For example the power suply that I had read the manual for could be set up to wait 20 seconds after the ignition was switched off before it shut down the computer, were the ITPS is fixed at 5 seconds. It's all a matter of what your particular needs are tho, and both options are very good.