In reply to StefanB:
the bike doesnt matter all that much unless you are doing really technical stuff or hardcore down hill.
up to a point, all that youll gain by spending more is better quality components (more durable, lighter, more shiny etc)
You have to go up large price brackets to get big differences such as a full sus xc frame, really good forks etc.
So within a certain price bracket you get essentially the same bike with a range of different levels of components e.g. shimano deore on a less expensive or a shimano XT on a better one. But the bike itself will be pretty much the same beast.
So ask yourself if you need a hardtail or a full sus xc bike? If you want to go full sus xc then yes you probably are looking at about a grand and a half, but for half of that you get a really good hardtail, which to be fair can do quite a lot.
You do loose a bit of traction with a hardtail compared to a full sus and yes you may be a bit slower than a good rider on a full sus. But as you are going from an old piece of poo to a brand new modern bike then youll probably get a lot of benefit anyway.
A lot of it is technique, which you can improve regardless of which bike you have.
I managed to keep up with two friends on giant anthems whilst I was on a kona hardtail in Afan last summer, they were better down the rough stuff than me and I was a tad better uphill - no idea why just technique?