re: Anyone know about a 12V input ATX power supply?
Assuming you're trying to do what I think you are, the easy answer is to just get a standard 12 VDC-to-110 VAC inverter and plug the computer into it.
The hard part is finding one big enough. You can get inverters that will power a laptop at any automotive store. But a 200-400 watt PC?
Any outfit that deals in solar panels or any form of alternative energy will be able to point you to large off-the-shelf inverters. They are needed to run household current off of battery packs. The big ones I've seen, though, aren't cheap.