So It's been about 7 years since i've Mountain Biked (thats what college will do to ya). But now I have a good job and am actually making money (yay) so I can afford a decent bike (not to mention I want to get off my lazy @$$)

Anyways I've read the FAQ so I have an Idea of what Im looking at (theirs a few bike shops around where I live, one sells Trek and something else, the other sells Giant,Surotta,Surly,Cannondale. I've looked at a few Treks (but got weary after reading a thread on here about the frame cracking). (I think the Treks I looked at were a 3series and a Wahoo)
Anyways Im looking to Keep the price under 650$ but onto my questions:

Aluminum Bikes: It seems im pretty much stuck with this.....which is fine but I heard that Aluminum fatigues very easy? Is this still the case? The shop-owner told me that they've pretty much fixed that problem but i wasn't so sure if he was lying or not. What about cracking/bending, is that covered under warranty?

Brand: I literally know ZERO about the brand, I had a Steel Giant (that was old as hell....but never really had any issues) but it was a steel bike and heavy as hell. What brands are probably best for someone that is going to do some trail riding (nothing to rough, but still.....decently hard terrain, no jumping off cliffs or anything ) With a budget of around 650$?

Also random question just for fun: Do those 6k bikes that companys offer (dont worry....im not considering one LOL) but.......can you "notice" a difference between something thats like 6k.....and like 2k (im just curious for you MB enthusiasts)

Thanks!