Until I have read a detailled test made by a reviewer I trust I think the improvement of the D300 on the D200 is about 1 stop. But Ed at Nikon Cafe is claiming a much greater improvement in this thread at Nikon Cafe:

“Been shooting a D300 all morning.

There is most definitely, absolutely, positively, leave-there-no-doubt-whatsoever a HUGE improvement in the RAW data from the D200 to the D300.

I would say the D300 comes close to par with the 5D. And before anyone challenges that, I have over 20,000 high ISO images worth of 5D experience. I also own 2 D200s.”

and

“They did something! I can hardly believe it.

I would guess it is improvement in the quality of what they capture and more likely they are removing noise in the RAW data.

In ACR, they behave a lot like 5D files. Even at ISO 3200. I’m not just being a fanboy here!

I saw this in Gale’s image. That’s why I bought one. You have to try this stuff in light YOU know. Shooting at things you have shot thousands of times before. Then you get goosebumps!”