Keep in mind that around $500 is basically beginner level- so perhaps you should consider bikes up in the $800-$1200+ range? Yes, test ride everything and try different LBS's- some brands to be sure and try if possible- Trek, Specialized, Kona, Giant, Cannondale- do some research on the respective websites and find the bikes in the price range you want and then go in to the shop loaded with questions and a list of what to ride (basically I agree with pinkrobe and I am just saying it a different way I guess...) I most definitely agree with the stumpjumper
as to disc brakes, skip getting them stock and put them on yourself- you can get the same brakes, save money, and have a back up pair!