There is no reason why you can't buy the Nikon 24-70mm F2.8 and use it on the D300. If you are thinking of upgrading to full frame at a later date, then it makes sense to buy those lenses now. Also, consider the AIS lenses which were made for film cameras, as most of them will work just as well on a D3. I use 400mm F2.8 AIS lens on my D3 with full auto-focus no problems. For infomation on other Nikon lenses see Ken Rockwell's site, a link to which I've posted below.
http://www.kenrockwell.com/nikon/nikkor.htm