Depending on the width of that hood or your willingness to build another one and work to rig a non-direct application.
http://www.prolighting.com/4fo3t5inst12.html
Thats 3 54W T5HO bulbs. They are saying the standard reflector comes in at 91%, though you can upgrade that to 95%. One solid reflector over individual ones though.
http://ahsupply.com/
I would look at their 54W kits. nominally 4 foot length. Each with individual reflectors. a two bulb kit costs $90. 3 kits would be in the $300 range but require some work on your part.
Honestly though, if your heart is insanely set on 500W, you are looking at MH and learning to love algae.
Oops, found one more
http://www.prolighting.com/6lat5flhibay.html
That will put in 6 54W T5HO. They will fill it with bulbs for $30, costing you $180. You just cover shipping and building the hood/mounting/outlet wiring.
you can also check this out , I also agree with the above post if your handy retro your old one for T5's, Ballast end caps and reflectors all you need they produce very little heat so im sure you wont need any fans
http://www.4fishtank.com/light fixtures.htm
GL
i catalina aquarium lighting. i have 3 lamps and they are great.
they have a 5 yr warranty on the ballast and its made in the use, sac CA to be exact.
http://www.catalinaaquarium.com/index.php?cPath=71
but if your handy then get some retrofit stuff and save yourself a bunch of money. dont forget the refector.
Thank you for all the input but I do not want to go with a retro kit because I want to do away with the hood/lightbox all together and have some free standing lights because of aesthetic purposes.