bhammer, I think everyone is poking at a single 500W bulb here. There's several factors to look at. Plus, I'm not even aware that there is a 500W system out there. I know about 400W, and I know about 1000W, but never heard of a 500, but that could be just me.
500W is probably a good choice for your tank, IMO. However, MH bulbs only put out so much of a footprint, so in order to cover your tank you'll probably want to split it between 2 bulbs, meaning you have 2 250W bulbs to create the 500W of total light you're after. Like Larry said, 175s could work as well depending on your bulb choice.
As for needing supplements, I think that's really personal preference. Bluer (higher K) MH bulbs have the blue color spectrums within them, so it's not necessary to have any supplementation, just many people prefer to have it as well so they can create a dusk-to-dawn effect by having the actinics on first for a little while, then the MH, then reverse the order when shutting down for the day.