Plenty of good advices on upgrading your MTB... If you're interested in entry-level road bikes, take a look at the Trek 1000, Fuji Ace, GT ZR5.0, Giant OCR3, and Bianchi Brava. Except for the Fuji Ace, the others are slightly more than $500. All of them are really similar with Sora components. The Brava and Ace are steel, while the others are aluminum. However, it's the time of the year when LBSes mark down 2001 bikes as they receive the 2002 models, so you might get some good deals.
Of course, the cheap way is to get a used road bike to see if you're really into this potentially expensive sport. However, it didn't work for me as I spent months unsuccesfully looking for a bike that fitted me well. Frustrated, I just bought a new road bike and have enjoyed it tremendously.