What your primary concern should be after you decide what kind of bike best fits your needs is the frame. Components can be upgraded piece by piece afterward as a sort of 'pay-as-you-go' deal, & old components can be sold off if they're still in good condition at the time of upgrading. For the $1K range, I've heard good things about the Giant Yukon FX. Solid full suspension frame & components that you would find on a $400-500 bike, for $850 new. On the higher end of the $1500 cap, you've got the Specialized FSRxc, which I've heard great things about. There are limitless options out there, you just have to determine what kind of bike you need & try them out. It sounded like you were really interested in XC, which is what both those bikes are for. If you find that you're more into downhill or dirt jumping, those categories also have very different bikes made specifically for them. (be forewarned, though--downhill will require a much higher price cap)