If you're leaving your saddle in that position while you're mountain biking, that means you don't need serious suspension, etc.

If you're railing so hard that the saddle gets in your way, then I'd look at upgrading.

My mountain bike was around $500 or $600 brand new...I originally picked it up just so I could ride the canal towpath with my wife when she wants to ride...that being said, all I ride are paths like you pictured, so there's just no real reason for me to have upgraded to anything else.

It was kind of intimidating to go look at mountain bikes without doing a ton of research first...but in the end, I'm happy with my purchase.

I would say if you do decide to go get one, read the reviews and some forum posts about it and see where you end up budget wise.

Basically if your budget is $1500, spend all of it...and if your budget is just $600 there's no reason you can't get a nice machine with that as long as you don't do crazy trails/downhill.