From my limited knowledge of audio system, I think you'll be fine with 500 watts. Basically (or atleast the way I see it), the more watts you have, the louder you system will be.
An average home entertainment system is about 1000 watts, so 500 in your car should produce decent sound.
On the other end of the scale, Bentley's has just released a 1100 watt sound system ... Overkill in my opinion
8.5" subwoofers in the doors? 8.5" = 8.5 inches. I don't think that would fit in your doors, but I may be wrong. Sub's are basically for Bass (deep) sounds.