UPDATE: THe 6 Core Apple Machine can take up to 32GB RAM. See my article about building a high end photo editing workstation.
Apple launched their new MacPro models today, including the 12 and 6 core models. The 6 core 3.33Ghz is the one to choose for a high end photo editing workstation, with fastest clock speed and memory speed, without the system overhead of the 12 cores / 2 processors that most applications will not use. However Apple appear to have limited the maximum amount of RAM available for the single core processors to 16GB. The single processor machines have 4 memory slots, the dual processor machines 8 slots. Apple say that the 8GB modules will not work in these machines.
Two of the biggest companies selling after market RAM are disagreeing about whether it is possible to get around this. OWC are currently saying they think it does have a 16GB limit, but in a conversation with Crucial earlier it seems that they are guaranteeing that their modules will work up to 32GB! Great!
Here is the conversation:
Welcome to Crucial Technology’s Expert Online. An Expert Online agent will join you in a moment… Please do not begin typing until you have been connected to an agent.
Hello, my name is Sean . How may I help you?
Wayne Grundy: I have just ordered a MacPro 6 core from apple. I am looking for the memory to take it to 16GB. I was expecting to see 4x4GB modules for this machine – as recommended by Apple. Apple say that this machine has a limit of 16GB, but on your site you are selling 8GB SIMMS, and say that it can go to 32GB RAM – I think this must be wrong???
Sean : One moment, please, while I review your inquiry.
Sean : Okay Wayne, looking at a Mac Pro 6-core system’s memory specifications. I see that it has four slots and that each slot can definitely handle 8GB each for a maximum memory of 32GB total. We guarantee that for that particular system. I’m sorry if I cannot draw you a comparison from what Apple say’s in reflection to what our website say’s but we definitely guarantee that system maxing out at 32GB. If you are still too concerned about that then my absolute best suggestion would be that once you receive that Mac Pro 6-core, you run our Crucial system scanner on it. It’s right on our home website and is a free download, what it does is scan that system’s information and gives you a list of all of the guaranteed compatible memory based off of that scan result.
Wayne Grundy: OK I will do that – I know that OWC are saying that the max is 16GB too see here MacPerformance Guide
Wayne Grundy: I think you need a definitive answer otherwise people will be ordering from you and they may not work!
Sean : That’s the best answer I can give you above Wayne. If for some apparent reason the configuration would not work for you a return can definitely be setup for the memory because it would be under warranty. But it still stands that we guarantee the 6-core going to 32GB. We wouldn’t guarantee that if that did not work.
So there you go!! – You read it here first!
6 cores, 3.33Ghz, 1333Mhz memory up to 32GB – Lightroom will fly!
Crucials 8GB SIMMS
Wayne John Grundy's PhotoTech Blog 
I spoke with the Micron engineer who personally tested the 4, 6 and 12 core 2010 mac pros on August 9th with regard to memory compatibility.
After a long conversation and some additional research he confirmed that in fact the six core Westmere Mac Pro is a dual channel memory system (integrated in the chipset) and NOT a triple channel system like the 2009 Nehalem chipset Mac Pro systems were. Therefore the best way to populate the 4 slots is to populate them all with identical memory which will run in effect in dual channel mode.
In addition. He stated that he in fact tested 8GB sticks of 1333 ram in the six core system and it did recognize and run the 8GB sticks. In fact Crucial has these 8GB sticks (albeit very expensive) listed now on their site as compatible with the 2010 Mac Pros.
I certainly wish I had this info when I placed my memory order with Transintl. Nevertheless, it seems to indicate that the max amount of ram for the six core is looking much better indeed. By the way, the board is a derivative of the Intel 5520 for anyone who is interested.
“Unlike Core i7, however, mainstream desktop/mobile Westmere chips will not feature a QPI bus or a triple-channel memory controller. Rather, we’ll revert to a dual-channel controller and have no QPI clock to worry about”
Thanks xgman, that is great input. It makes sense to buy 16GB as 2x8GB and then upgrade to 32GB when the prices for that memory have fallen.