I went through the same dilemma myself.
I came to the conclusion that my D80 would probably devalue by about another £100 from the current secondhand value of £340 - £380 in about a year to around £250. This is allowing for the fact that there will probably be a replacement model by then.
The Current cost of a new D300 is around £1050 if you shop around.
I reckon this will come down to under a grand fairly quickly and probably be selling for around £899 in a years time.
Logically speaking the D300 will certainly come down by more than the D80 because of its high starting price.
Therefore no need to panic into selling your D80 now as the cash difference in upgrading should be less in a years time.
However - I thought sod logic and sold my D80 on here and rushed out and bought myself a D300.
My D80 was my first DSLR and I am still very much a learner.
I thought the D300 would be a lot harder to use, but I find it a lot easier somehow. It seems a lot more intuitive and the manual is great. More than 400 pages and all in English, but very easy to read and well thought out.
I love it.
