Depends on what you shoot.
If you shoot fast action or events where you take a lot of photos quickly (ie. weddings), you want something that will read/write very fast:
http://www.sandisk.com/Products/Item...Flash_4GB.aspx
If you shoot studio, portrait or basic events (birthday, holidays, etc...) then maybe you don't need top speeds:
http://www.sandisk.com/Products/Item...tFlash4GB.aspx
Do the fast cards work in the D300? - They sure do, they're very fast and if you shoot that way, they are worth the extra dollars.
Joe