I upgraded from D80 to the 35mm DSLR D700 (which is way above the D300 in ability).
So I can't give you any real proper comparisons on performance.
I only ever once used a borrowed D300 to test a new 24-70mm F/2.8 lens before I purchased it, so not really a proper comparison or evaluation - but from what I did try, the D300 in operation was mostly identical to my D700 - which means that it is very good. It will however be worse in low light conditions, but you'd expect that - and what remains is still very good anyway.
But what I can say is that the handling and ability to customise the D300/D700 is worth the expense to upgrade. They are much higher quality. I particularly like Active D Lighting too. The ease of use is much better, all the major functions are right where you want them, they don't have silly portrait, landscape or sports mode dials- and they are far more rugged cameras.
You might also consider the D300s if it is in your price range, it will save you money on memory cards. Remember that the D300/D700/D3/D3x use compact flash, while the D80 uses SDHC cards. The D300s can use both. While not a primary worry, the D300s can also do 720P video if you want to experiment with that.
The battery grip is very much worth the expense - it improves the balance and feel of the camera, and when coupled with the EN-EL4a battery and BL-3 combination increases the frame rate by a very useful amount, getting you up near the D3's speed.
The D80 is still good, but you can reach its limits - while the D300/D300s/D700 are far better. I don't know what lenses you have at the moment - but if you have half-decent ones, the D300 will complete the package.
[Edited 2009-08-27 03:43:15]
[Edited 2009-08-27 03:44:57]