Just realized this quick comparison:
Base model 8-core with only upgrade being 16 GB RAM = $3799.
Base model 4-core with only upgrade being 16 GB RAM = $4349.
May not take advantage of 8 cores right now, but if I can't get more RAM with the 4-core, it doesn't seem to be a viable option.
If you can "make do" with just 12GB of RAM then you could get a Quad core with OWC memory for under $3K.