They are both big hitting, long forked steel frame hardtails, I would have thought the overlap was huge, and weights similar.
The 456 certainly doesnt fit in with the more XC orentated ride the OP was talking about, only in as much as he could just XC up the Alpine and not bother changing at all.
My opinion would be to do just that XC what you have, escpecially if you like it, run some light tyres on it. Then you still have it for when you get the chance to ride it for what it was designed for.
If you fancy an XC bike and dont want to drag the wieght of a heavier duty bike about, there are loads of choices. Scandal, Rock Lobster team tig, for eg.
In answer to your question, god knows what its worth as bike or in bits, depends on the day from my experience of both the classified and Ebay.
edit, you just beat me with your post, keep it, XC it, and do what its made for when you get a chance, as it will cause no end of agro selling then buying again, and you would be lucky to break even on it all.