I have a question for you.
Do you think you will ever:
1) Want to use the PC with the engine not running
2) Have the PC running when you start the car (cranking)?
If your answer to either of those is "Maybe" or "Yes", then please, save your money, and don't get the ITPS. It can't survive the crank, and more than likely won't power your PC with the engine off.
Now, onto your question, which relates to the Opus, Carnetix, and the ITPS.
You have 3 wires, one direct to the battery (with a fuse close to the battery!), one direct to "ground", which is basically the nearest bit of your car chassis to the power supply, and a switched 12V. I can't recommend you hook it up to your cigarette lighter, however, do it properly and connect it directly to the ignition, which will probably be a yellow wire under the dash, or there will be a wire which does the same job to turn your stereo on/off