At 500, you won't be choosing between low en components and low end frames, you will get both....I was in a similar position a few months ago, and quickly realized I needed to up my budget a few hundred to 700-900. In that range, the best racing bike based on reviews I've seen seems to be the specialized Allez...It has a well designed frame, and functional components.

(However, I'm not looking to race, so I'm looking at slightly more relaxed models.)