Save Your Money

$90. Good Lock.
$90. Helmet
$75. Panniers
$50. Rack
$30. Lights
That leaves $365 for a bike at max budget and other stuff I didn't mention. I recommend saving your money by using what you have now.

If you have upgraditis or yearning for the new bike tailwind, then I suggest the used market. Pickup a decent bike for half of what remains of your budget so you have a bit of leeway for any surprises.

http://seattle.craigslist.org/sno/bik/1981702032.html
http://seattle.craigslist.org/see/bik/1981676008.html