A good car battery does have an amp hour rating although its not usualy posted on it and they can run a fair sized load for a short period of 5 to 10 minutes with out damage but like audioguru said deep discharging a car battery is hard on it and can destroy it.
My dad used to buy the cheapest crapiest batteries he could find for the farm equipment. (because they constantly went bad, so why buy good ones?
)
If they got ran down only a few times to the point of needing a jump start they would go bad!
However a good quality battery can take deep discharge occurances better as long as they get recharged immediatly. But still it is hard on them.
I personaly buy rather expensive dual use marine batteries for my work truck because I do run a big inverter (2000 watt cont 4kw surge) and fairly heavy loads for extended periods off it at times.
These batteries do have a good deep discharge capacity and a good cold cranking amp capacity too. They are like a starting battery and a deep cycle battery combined. But still they do go bad within a year or so from being run down too many times. (I love 60 month waranties!
)
I have been around many higher wattage car audio systems and the ones that run for an extended periods of time use a second deep cycle battery or sometimes several. They can take deep discharge and not be adversely affected.
Running your 500 watt system should not be a problem for short times but it very well could ruin your battery if you get carried away too many times!