what guver said, also try these links
http://www.marcee.org/Articles/PCPowerSupply.htm

on the menu to the left all the way down
http://www.rcbatteryclinic.com/

there is also a sticky in the battery section on how to do it. Mainly you are looking at the weight of the computer PSU. 1 or 2 lb PSU's are junk, but might still work for you if you are running like 50% of the max amps on the 12v rail. The 5 or 6 lb models are good for what they are actually rated for.