Mac Pro upgrade path I'm considering a Mac Pro purchase but cannot afford the one I really want. 
The question is this; Can the cheapest Mac Pro 5,1 single quad-core Nehalem 2.8 GHz be upgraded to the dual six-core Westmere 2.93 GHz at some point in the future?
If not, what would be the cheapest one that could?
Short answer:

The lowest end Mac Pro that can be upgraded to a 2.93 12-core Westmere is the 2.66GHz 12-core Westmere.

Long answer:

The single socket Nehalem and Westmere Mac Pros literally have one socket. Aside from that, most of the lower end configs ship with a 1066MHz memory bus instead of a 1333MHz bus. In order to upgrade to a part that uses faster ram you will need to replace all your ram, and probably solder some resistors to change the clock multipliers on the board.

Without SMD soldering you will need to match the bus speed of the processor, which means if you have a 1066MHz memory interface the highest end part you will be able to install is are quad core 2.66GHz Xeon E5640s. There is also no guarantee the firmware will actually initialize it correctly, though odds are good that it will.