Mountain Bike?

I've been enjoying my bike rides so much recently that I have this urge to try out the Mohican MTB trails. Of course I've run most of them countless times but I think it would be fun to do them on a bike. There's just one problem. My mountain bike is really old and is not equipped to handle real trails. First of all it has no front shock, nor does it have disc brakes - two things I think would be very important when riding trails real trails. So of course I've been looking around if there are any good deals to find on a entry level mountain bikes. I've been surprised at how reasonable some of these bikes are priced. The following are the ones I've considered - all between $450 and $550.













Felt q520 - $470
















Cannondale F8 - $550
















Specialized Hardrock (disc) - $470
















Trek 3900 (disc) - $530

If anyone know anything about mountain bikes I'd love your input on which brands are good/bad.