Can I get anything good with $400?

I've been told I should stick with a hardtail if I'm not going to spend more than $700. The terrain I ride is in wooded areas, elevation is random but most of the time I am going downhill at a slight angle or on level ground. When it comes to climbing, its only on occasions and they are really short. The types of obstacles I encounter are consecutive roots and rocks which would make it impossible to gain any speed or ride comfortably without some sort of suspension. Some logs just small enough so the tires can roll over them. Then there are some fun stuff that my local trail features such as a dirt ramp as you go downhill, the ramp makes you jump over the small creek. It isn't too extreme though, I see a downhill bike unnecessary, the creek you jump over is only 4-5ft in width. Then there are some other technical features such as a rock garden, a part of a trail where your hitting roots non-stop, and some very steep hills that I wouldn't dare myself to go on (on my cheap Walmart bike).

This summer I have learned a bit about mountain bikes, I understand how air shocks work, length of travel and what the settings do. I also learned about frames, and why it is important to get a good one (I was surprised that metal could just break during regular use on bikes). Some things I do not quite understand is drive train quality, geometry, and chain stay length. Also same with the weight of the bike, and rotational weight and how big of a difference it makes.

Another thing is, I don't know if I need a hardtail or a dual suspension for the types of trails I ride. I've never got the chance to ride a proper mountain bike, but from riding my Walmart bike with just 1 inch of travel its just impossible since the roots and rocks slow you down that you are better off jogging the trails and can finish them quicker without the bike.

I'm looking for a bike thats going to be the best for the money, right now I found the diamondback hook on sale for $420 on ebay. Some other $400 bikes I've seen only have XC forks with travel under 100mm. Any recommendations? A friend recommended me this bike http://www.bikesdirect.com/products/gravity/29point4-29er-mtb-eoy.htm but that is a bit off my budget. Should I just save up and get that bike?