While I will certianly defer to those members with more experience...I will throw in my advice based on my recent bike search.

You can get a very nice hardtail in your price range. However, bikes in that neighborhood with disc brakes are certainly going to have some compromises in other areas. What you need to figure out is where the company cut its corners. Usually components and wheels. Components can always be changed but you are stuck with the frame. If you really want discs in the $500-600 range then you have to accept lesser quality in other areas. But it is probably easier/cheaper to get them to begin with than upgrade brakes, hubs and possibly rims later.

btw if you do a search, many recommend the Giant Rainier with avids for around $700. I have no personal experience with it but a lot of other people seem to like it. If you cant scrape up the extra $100 or so you can always look out for last year's model or a slightly used one. Good luck in your search.