What do you have for lenses? I ask because the upgrade to a d90 or d300s is only worth it if you already have a decent set of lenses. Upgrading the body and using a shitty lens will still give you shitty photos (quality wise, you'd be surprised how much good glass makes a difference!).
That being said, if you go ahead with the upgrade, then here are my thoughts (i quoted despite b/c he brings up some good points, however, I don't agree with all of them).
First off, higher FPS + better focusing system (of the D300s for example) is nice if you shoot sports like football or lacrosse, but if your shooting something like skiing where its just 1 person in a simple environment, the nicer focusing system of the D300 probably wont make too much of a difference. Also, having 5 fps is more than enough for skiing.
The quality high ISO shots are always good to have, but i find that, at least when i shoot, i dont go above 800 or so max unless i'm shooting for the newspaper (school paper, and hopefully real newspaper this summer) in a really poorly lit enviornment. I'm not sure how the d60 is at high ISO, but do you find yourself limited by ISO? If so, the d90 high iso performance is almost the same as the d300 and pretty darn close to the d300s.
Also, what aspects of the CLS does the d300 bring that your camera cant do?
My vote as of now goes to get more lenses. If not, d90 or d300s is nice. I really like the professional feel of the d300 but i'm not so sure its worth the extra money, especially since the d90 is a killer camera!
"When people say you have a one track mind, tell them they're only half right. Skiing leaves 2 tracks!" - Seth Morrison