From my experience going to 6Gig is workable on this system. Going to 8 is very risky.
So far I've found the slower clocked CPU based systems work better than the higher clocked CPU's of this model line when pushing the limit to 8gigs. I suspect the controller logic runs into trouble with the higher lines it has to read on faster CPU based systems. In any case its not worth the hassle so I stop at 6gig when upgrading any of this model line
Here's another reference: Mac On Steroids
Referring to the following pages you'll see Apple doesn't support more than 4gig and many third parties likewise stop there as well.
Apple has updated their spec sheets before when newer memory chips or firmware updates allowed more memory to be supported. Then again, timing is everything. As this is such an old system Apple may have decided not to update the spec sheet (remember they need to do a lot of testing before they bless more memory per a given system). So these sheets could be very dated and Apple never took the time to do the testing hence the sheets show the limit is still 4gigs yet 6gigs work and 8gigs is going too far due to hardware limits (memory controller on faster CPU based systems).