hey audie,
Its great that you want to get involved in cycling.
Road bikes are often very expensive (i would know having been in the market for a new bike for about 6 months) but your budget is not unrealistic.
Assuming that you mean USD$500, there are a few options available.
The most important thing is the fit, so whatever you decide to do with the options available, make sure the bike fits you!
the first option is buying on ebay. this is often not suggested because of size/quality issues, but if you go to your LBS and get fitted first, so you know what sort of frame you need, there should be no problems size was. on the quality front (might be half a bike) try to buy within you area, so you can go and check out the bike before you buy it.
the second option is buying second hand. there are plenty of dealers around, and as long as the bike fits, and everything is in working conditon, it should be fine. A tip for this is to buy a bike that is post 01-03... that way it is highly unlikely that the gears have worn down etc.
the third option is to buy new. naturally, this is more expensive, though definitely possible. It also means that size and quality wont be a problem, because it's brand new.
If you want to race, then what you really need is a bike with Shimano Tiagra or higher. because this can be more expensive (cheapest i can find is USD$845) one of the first 2 options would probably be your best bet.
On actual models, i sugest that you go down to your LBS and ask for some advice. Give them your budget, and listen to what they say.
A few more tips are:
Dont buy a carbon frame for a while - they're WAY too expensive, and if you drop them they crack.
Do use clipless pedals and bike shoes - it will make life a lot easier
Dont become materialistic with helmets/shoes/bikes - the fit not the brand is the most important
Do wear a helmet - your life is worth more then your dignity
Only buy what you are comfortable with - stick to your budget
Do enjoy cycling - whats the point of not?
hope this is of help,
john