[JavaBrewer]

Find out what size frame you need and then hit Craigs list. Nearly all department store bikes are complete garbage. As stated you need to be looking in the $500-$600 range if you intend to take the bike in the dirt. Even then you will be breaking things and the bike will need regular adjustments and maintenance. Dirt, rocks, bumps, jumps, water, etc...are very hard on bikes. At your price range the wheels will be the biggest issue. Cheap ones are knocked out of balance quickly. With Craigs list you can usually score a MTB that's a couple years new for 50% of the original price so keeping your $500 budget will get into something that's more durable and serviceable.