Which processor would be better for running Avid Xpress Pro + Mojo? Also, will a dual processor motherboard run with only 1 of the processor slots filled until I get the 2nd processor chip?

i would go for the Xeon. Mainly because Xeons have amazing stability(as you would expect with a server chip), and when editing, stability is very good so you dont loose everything. also a large portion of the actual video processing is done with the Mojo, so speed is not realy required that much. I would prefer stability over speed anyday.

A mainboard with dual processor slots should be able to run with just one processor, you might miss out on features like dual channel ram etc.

One of the machines i currently work on is a dual xeon with a scsi raid and a canopusDVrex and that thing is rock solid as all hell. Definitely recommended.