One, that rating is the maximum amperage that can flow through the relay. It's not saying that it automatically flows 200 amps through whatever is connected to it.
Two, no electrical (aside from the actual starter) will draw anything close to 200 amps. That's huge. You could run ~2800 watts of amplifiers before you'd hit the 200 amp mark. My alternator only puts out 100 amps max. I don't think anyone even makes a 200 amp alternator from the factory. Some car audio specialists (Stinger, of course) produce high amp alternators, ranging from 135 to 270.
I don't think you need to worry about blowing that relay. I think it might be a bit of overkill for just your computer. An Opus 150W should draw a little over 10 amps. Even using a 400 Watt AC/DC inverter, it'll only draw about 30 amps.