You can buy a machine with a terabyte of RAM. If your check doesn't bounce, IBM, HP, Sun, et al. will deliver it via forklift tomorrow. However, that box will cost much, much more than 16 boxes with 64GB each. Those 16 boxes will also have more CPU, network cards, and aggregate bandwidth to RAM than the terabox. The big honkin' box full of RAM is in most respects an unbalanced system; you can do a lot more with a lot less money if it's acceptable to give up on the single-system image programming model.