Apple only supported 4GB FB-DIMMs in these models, for a total of 32GB RAM. However, the chipset was always suspected to support 64GB with 8GB FB-DIMMs, but everyone who tried this only had RAM recognized up to 32GB RAM. But today there's an interesting post on netkas of someone who used mostly 8GB FB-DIMMs and left two 4GB FB-DIMMs in the last two spots of the second riser. The apparent result? 56GB RAM in a MacPro2,1 Very interesting if it really works properly.