It's very possible to get the PC down to drawing only 2 amps. When my PC was in the testing phase, I had a Shuttle FV24, P-III 933Mhz, Maxtor 160GB, KB, mouse, 4 fans, Xenarc touch screen all powered by Opus PSU. The opus was hooked up to a indoor regulated PSU. The voltage was set to 12.5V. Playing MP3 alone draw only 5 amps. If you use older boards with older processors and laptop HDD and design your system for ONLY playing MP3s then you can get your PC down to drawing 2 amps. It could play for more than 15-20 hours on a healty battery and still be able to start your car.
Edit: Mine's been running for about a year now. Opus sometimes need to be reset (disconnect the main power for 10 seconds) Keyspan 4 serial hub to USB sometimes wont load and I had to reinstall the driver and reactivate the touch screen. Other than that the system worked flawlessly