Let me second Kius vote for the 24/2.8. I grew up on one as part of my triad of 24/85/200 and it worked great. It is a great lens, sharp and bright. Should be easy to find. I am using it as my wide angle lens for my D300 (some day soon I hope). My kit will be 24/58/105/180. I suspect that the 24 and 58 will be used most of the time.
While I might be tempted to look at a 20, I've never really liked the 20/21 focal lenght. For me it was either too wide or not wide enough, never just right. I had a 21/35/90 kit on my Leica Ms for years, the 21 almost never came out. Now I have a 15mm that I love so much that I am thinking about trading in my LTM version on an F mount. Problem is for me that the D300 does not have a real mirror lockup. But it will work on my Nikkormat and with an adapter on the Bessa R I have for my son. If Pres K would ever reissue his F to S (Nikon SLR to Rangefinder) adapter again I would jump on it. I find the 15 to be great. Really wide for those shots that need it.
B2 (;->