I have been thinking about upgrading to either a D90 or D300 (not the D300s because apparently the only difference is it does video) so would be interested in seeing the outcome of this.
I'd say that the bigger difference between the D300 and D300S is that the D300S has dual memory cards. Video is neither here nor there as, whilst it's not existent on the D300, it's terribly implemented on the D300s. There are a couple of other relatively minor changes. Dual card slots are nice to have but right now you pay about a £200 premium used for a D300s so you really need to want them. For what it's worth, a used D300 is looking a serious bargain right now. If buying new, the point is moot as the D300 is long discontinued.
Anyway, back to the question. It's definitely a big step up but don't expect your images to change much as the sensor is basically the same, though actually some people reckon the D90 deals with noise better but I think we're splitting hairs here, they're basically identical.
Where you'll see a big gain is in the ergonomics. The D300 is substantially bigger and heavier which means that it'll fit your hand like a glove. More importantly, all of the controls that you could hope for will fall easily to your hand. You'll find that compared to the D90, you're almost never in the menu system and even if you do need to venture into it, you can set us different settings profiles for rapid changes between shooting modes. It's a dream to use!
Other benefits, far better auto focus system, better built, weatherproof, higher frame rate, especially with a grip.
One thing to bear in mind, if you make the change is that you'll need some CF cards.
giving Macro ago in the very near future. The reason being is a friend is taking up photography and would like to buy my D90. 
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