First up word, the F80 is a crazy bargain at the mo. You can't use your current lens though.
second, I'm presuming your after the close in, out of focus background look? two ways of acheiving this - long lens or wide aperture (small f. no).
You probably want a general use 70-300 (or dig equiv), these cover a useful range but are generally slow (ie narrow max aperture) meaning you'll have to crank up the iso in poor light, plus to get blurred backgrounds you need to use it at the long end, ie stand further away which can be restrictive at the crag.
So also consider a shorter, faster (ie wider aperture) prime lens (non-zoom). Dave has been raving about a 50mm/ f1.8 on his nikon digi, plus these are fairly cheap. Great in low light, you can use closer in and still blur the background, the faster the lens the better the autofocus performance, plus they're smaller.
