Sounds like you're already working up to pretty hard-core mountain biking. Like the man says, for 600.00 you're going to get an entry-level bike that will probably make you unhappy in a short time.

For about 1000.00, you start getting into the range of really nice bikes. You can pick up Specialized, Trek, Schwinn, and a host of others in that range that are light, have good components, and will serve you well.

I had a Schwinn Moab III that was an absolute delight in that price range.