$500-600 is not a lot for the whole 5.1 setup, but it's more than a lot of people start with! In that price range, look into some satellite systems with sub included, I suggest going to some stores and listening to different brands and decide which suit you.
The other option is forgetting about 5.1 altogether, and build your system over time. You can start with 2 decent towers but no sub(stereo), you'd be surprised how good it can sound with quality towers. Other options being decent 4 bookshelves but no center or sub, which will give you the most basic surround sound possible.
That's a pretty big room, so I would suggest the route of less larger speakers vs more tiny satellites. Either way I think you'll be pleased, taking your real first step into audio/surround. One rule of thumb is to budget the same amount for audio as you have on your TV, as 50% of the movie experience is the audio, seriously.