For amateurs (not referring to skill, just people who do photography not as a job), there is usually not much of a reason to upgrade, because any upgrade is just a liability since it doesn't make you money.
That said, upgrading does give one the joy and motivation to shoot. For me I'd say that the D300 isn't worth it now. I'd go for the D7000. The D90 has the same sensor as the D5000, and if the sensor wasn't that old I'd upgrade to it (which I did). Right now imo the D7000 makes much more sense. It'll keep you from upgrading for a longer while