The OWC 8GB modules have been running flawlessly for two months now— highly recommended.
In the quad-core Mac Pro Nehalem (see review), the four slots accepts four modules, maxing-out at 32GB of memory. But unless you really need 32GB, 24GB is better idea, since triple channel memory is fastest. I use 24GB in my 3.33Ghz Mac Pro.
For my 8-core 2.93GHz Mac Pro Nehalem, I use 48GB (six 8GB modules), which is also optimal (triple channel memory, 3 channels for each CPU).