OWC has memory for the 2010 Mac Pro available. The 24GB and 32GB configurations for the 4/6-core Mac Pro models are not listed, but as soon (and if) OWC verifies compatibility, those options will be added, so I’m told.
If you’re investing in a Mac Pro, don’t mess around with 8GB— it’s OK for some uses, but if you need more, you’ll have to remove it and then buy 4GB modules. So start with 12GB, and for big jobs, start with 24GB. I personally use 48GB (six 8GB modules), and Photoshop Cs5 regularly scarfs up 10-15GB for itself, so 24GB is my absolute minimum.
Remember, you could have a 50GHz processor, but if the system gets low on memory, you’ll be running at the speed of the drive with virtual memory, not the speed of the processor, and all those cores will be idled.