If you come from a mountain bike background I'd recommend looking at cross bikes. They give you a lot more options than a road bike. Gravel, grass, trail, road, etc. They don't do any one
thing really well but they do a lot
of things pretty good. You can put some narrow road tires on and ride asphalt or mount up some wider cross tires and hit gravel.
As Snoop was talking about the newer road bikes have an integrated shifter/brake lever, aka "brifters." You press the paddle down one way and it drops gears, push the brake lever combo to the side and it shifts up. To brake you just squeeze the brake lever like normal. Its really a pretty slick set up compared to mt shifters/brake combos, until it breaks...
If you look on CL you can find some decent road or cx bikes in the $400-$600 range. If you see something you like check out http://www.bikepedia.com/
and it will give you the basic run down on the bike. You might have to sort through the color combos to find the right year you are looking for.
Be sure to find a good fit. One brand's 56 cm bike may feel a lot different than another brands. You can adjust the stem length/angle just like a mountain bike and it gives you some flexibility in the reach department.
Finding a used bike that is in your price range and that fits is a PITA. Be sure to take your time and don't settle on something that does not fit or does not meet your needs.