Unless you go with the new 6Gb/s SAS then SAS is just 3Gb/s like SATA II is, like taltamir said above. You will get a bit more throughput to the individual drives, I think, but not all that much. IMO you're better off with a SATA RAID array (tons of space, compared), or if you have the money SSDs (blazing fast).
EDIT: SSDs excel at random read/write, while SAS are better at sequential ops. If you're constantly copying large numbers of full pictures back and forth then you'll probably be better off with SAS. If you're editing pictures and moving them around one at a time (outside of when you dump upload them from your camera) then SSDs seem the ticket.