The two generations have different memory controllers.
The xw8600 and the xw6600 have the same memory controllers.
Officially the xw8600 supports use of 8GB sticks, and the xw6600 does also (but not officially), assuming they are the correct type of 8GB sticks.
No go on the xw6400 or xw8400 with the exact same sticks that worked in the xw6600 and the xw8600 during my testing of this.
