I think just asking if there will be a difference in image quality is probably missing the point about upgrading.
If you get the D300, you are buying into a new system which will increase your options when it comes to what and how you take a photograph and the type of results you can obtain.
Images I took with my old D50 at first glance dont seem any better than with my D200 or D300. But, the D300 will enable me to tackle more challenging shots with a better keeper rate, which is good for me as I experiment more and attempt shots that I wouldnt have done with the D50/70s/80/200 that I had before.
Printing the results out of both at A3 will show a small but significant difference in resolution which is in keeping with the larger sensor and better processing of the D300. I also notice better colour rendition, more detail in shadows and an overall sharper image. Coupled with a good lens, I notice shots just look better, and the D300 has the ability to reduce chromatic abberation that shows up in some lenses. Any lens that I used with my D200 that produced even small amounts of CA now dont.
In one sense then, the D300 has improved all my lenses
Dont get hung up about the technical side of how much better the higher ISO capabilities of the D300 are over the earlier cameras. All I know is that when I need to use a high iSO setting I get good pictures and on the rare occasions I set ISO to auto ( something I would never dare do with the D200) I get great low noise shots at up to ISO 1000. I dont generally need to go over that but I have done so and been very pleased with the results.
I dont see any noise at ISO 200 either, but if I do, I can always drop the ISO down lower. However they acheive this drop in ISO, it works!
I recently posted a link to DXOmark. This is a utility that will actually compare the ISO of different cameras in a subjective way not the usual sweeping statements of how good or bad a camera is in one persons opinion. It will also show the true ISO as opposed to the ISO the maker claims their camera acheives. For example, a D200 at a setting of ISO 200 is actually 195. A D300 at ISO setting of 200 is actually 145. Not far off the base setting of ISO 100 of the D200 and worth thinking about when critisizing the apparently high base ISO capability of the D300!
Anyway, heres the link again,
link here and another link to explain how it works,
here
And finally ( always wanted to say that) If you want a new body and can afford it, just get it! I never need to analyse my reasons to buy something new if I have the funds. I probably hadnt learnt to use every feature of the D50 before I upgraded to the D70s, likewise, all my other upgrades to my current D300.
I sometimes leave some dinner on my plate before I have my pudding too!
Allan