Here's another option to consider:
Buy the single 6-core 3.33 and load it with 3x8G of OWC ram. (Buying your RAM and SSD from OWC too will save a ton.)
I agree, right now this seems to be the way to go . As for upgrading to more cores, time will tell if there'll be a need.
Right now, 2D still editing is mainly about RAM and processor speed.
How do you set up a mac for photo?
It's been linked to above - you really want to visit macperformanceguide
and do some research (the owner is affiliated to OWC, he mentions it everywhere, but keep that in mind).
I'd add that any current SSD with Sandforce controller (OWC, OCZ Vertex 2
etc.) is supposed to work well on a Mac and last, but noone knows for sure just yet.
Imho, concentrate for performance on your boot+apps drive array, and a scratch and project drive array; for storage of infrequently used files and archives just stay away from USB and you'll be fine.
The whole PC industry didn't manage to introduce something better than Firewire 800, or faster than eSATA, for external hardware (on our level), so I wouldn't hold my breath re. lightpeak and even USB3 if you need a solution within a year or so.