So would you say the noise levels are higher above ISO 800 than my D90
They're basically the same sensor. I'd be highly impressed if anybody could tell the difference in noise, IQ, DR or whatever else. I quite happily used mine up to 3200, but whatever iso you're happy with your D90 at, you'll be happy at with the D300.
The d90 has the edge on image quality, but the d300 is better built and more robust.
The d90 isn't exactly feeble though. Why do you want to replace it?