Mac Pro purchase question

This so sounds like a newbie question, and I know that, but I gotta ask anyway.

I am a fine art photographer, and I currently shoot a lot of my work with a D3x and work with pretty large files as I often print to 40x26. I am currently using a very tired Mac Pro first gen 667 that i have kept alive for the past four years by upgrading the drives and running 18 gigs of RAM in it. It's time for an upgrade and I want to replace it with another Mac Pro for reasons that have more to do with wanting more internal drives and external monitors (yes, I have thought about a 27" iMac), any way, looking at the various version of the Mac Pro line up I am thinking about either a Quad Core 2.8 processor or the 8 core version. My budget would be tight with the 8 core and getting the quad core would allow me to really load up on RAM. It's $900 price difference between the two.

The question is, for a non-video guy like me, going from my current Mac Pro to the Quad core that will be a big performance increase, but what my gain past that with the 8 core.

worth noting, I usually keep my computers for 3 years minimum and like to get four years out of them if I can, so that would be the plan for this one.