I'm currently in the process of trying to put a DC/AC inverter in my car to power my M10000 in a casetronics C134 case. The inverter works fine, but puts a huge amount of noise on my audio lines. So much so that it is basically un-useable for playing MP3's. I bought a cheapy inverter ($30 for a 120W inverter @ Wallymart). I guess I got what I paid for. I'm going to try to return it for the more expensive version ($60 150W) which touted it's filtered sign wave output. My guess is that my el-cheapo is putting out some sort of very noisy square wave that's killing my audio.
I'm not familiar with the various "ATX plug-in" on-board power supplies (like the PW-70a), but my guess is that they won't fit in my C134 case (very tight). If you are using the C137, just check to make sure there is room above the supply. In the case of the C134, there's no room above the power supply. In fact, it's packed so tight, there's no room for ANYTHING else.
I'd check out some of the technical specs on the Opus 150W supply (www.opussolutions.com). His specs tell the story of what you should look out for when trying to select a power source for an automotive application.
Good luck n00b. This stuff is really fun.