You need to decide why you want a road bike. If it's purely for fitness, you're going to be more than fine on your current bike with slicks. You have to remember that road bikes may weigh less but they have higher gear ratios- so your knees will still feel you mashing up those hills pushing harder gears. Use a gear calculator to see what the lowest gear is on your current bike v the road bike you're considering. You may find that spinning up a hill in a lower mountain bike gear is better than mashing up a hill in a 34/25 for instance.

If your goal is to be able to ride farther and faster in a given amount of time with a goal of maybe riding 25-30 miles regularly, then I'd wait for a sale at an REI or see what your local LBS has lying around. You should be able to get at least Tiagra for $1000. An example:

http://www.jensonusa.com/Road-Bikes/...t-Road-Bike-12