150 F is 65.5 degrees C, which is definitely a little warm... Are you using stock cooling? If so, I'd definitely consider upgradeing it somehow, or rebuilding the carputer so it is in the cabin of the car (Under one of the seats or something) so the A/C cools it. Watercooling, though it could definitely be done, is probably best to avoid in a car. I wouldn't trust it to not spring a link.
If you're not getting any lock-ups or program crashs from the heat, though, you could just ignore it. The worst that can happen is your processor won't last as long as it could have other wise. If it would mean a lot of time/money to drop the CPU temps, and you don't mind possibly having to replace it sooner than you would otherwise, I'd say just live with it...
The hard drive, however, I'd make sure has several fans blowing air across it, or maybe one of those hard drive bay cooler thingies made for hard drives (check ebay). And I'd keep anything important on it backed up some place else.
Just my two cents (USD, of course

).