Vizio isn't too bad for that price range. Olevia isn't bad for that price range either. If you can, I would definitely recommend going to a Best Buy, Frys or Costco and looking at the TVs side-by-side to determine quality.
Samsungs also make GREAT TVs as well but in all honesty, I'm not sure if they will fit in that price range.
Obviously any TV you get should be 1080p. Nothing below that (not 1080i or 720p) is worth it IMO. These days though, I don't see that many HDTVs that aren't 1080p but since you're looking at the $500 price range, be sure to check.
Also - have you decided if you want a plasma or LCD? That's another decision you should make before you buy. I'd personally recommend going with LCD. Plasmas will give you more of a true black, making the color contrast better which is the one thing I love about them over an LCD. However, they're known for having more glare, higher amounts of energy usage and shorter life span which is why I decided to go with LCDs. When you're looking at HDTVs, LCDs are more common than plasmas anyways. LCDs generally won't give you that rich color that plasmas do because of the true black factor but honestly, once you take it home, you probably won't notice it. Just the two factors of longer shelf life and lower energy consumption made LCD a big winner for me though.
Gluck and happy TV shopping!