Moving Back To A Mac Pro / Processor And Graphics Options

I have just finished a seven month fiasco working with a refurbished 3.4 GHz i7 mid-2011 iMac, with the Radeon 6970 graphics card. I had originally purchased it to replace an early quad core 2008 Mac Pro. I had the iMac hot-rodded by the OWC folks to include a Mercury 480 GB SSD and eSATA port. That story involves multiple issues with failed components and a complete inability to install bootcamp, which was needed for work-related software that did not play well in a virtualized environment. So now, with the latest iMac hardware failure (Fire Wire port), I have thrown in the towel and will be going back to Mac Pro. (Takes deep breath.)The iMac is currently being used for work, my second job as a part time pro photographer (Photoshop, Aperture), some videography and transcoding and occasional gaming.I'm looking to hit performance marks for processing and video similar to the i7 and am willing to consider refurb or pre-owned machines.  New 8 core Nehalem and 6 core Westmere machines are outside my budget.  Definite upgrades I intend to add are:

a. 512 GB SSD as primary drive for OS X

b.256 GB SSD for my Bootcamp installation

c.12GB RAM

d. 2 Seagate Momentus XT 750 GB drives in RAID 0 configuration

So, my questions are

1.For real world performance, particularly with regards to my photography work and video, what core processor combinations (going back to 2008 or 2009 used machines could be considered) are likely to yield EFFECTIVE processing power close to the iMac i7? Among recent vintage machines, it seems that a quad core 2.8 GHz would be reasonably close, but I find it's difficult to translate Geekbench results into my needs.

2.I'm having a real hard time with my video card options. It seems that potential choices include the 5770, 5870 and 6970 cards.I'm not overly concerned about the price differences, but would like to find the best performance/value combination, given my needs. 

Info:
MacBook Pro, Mac OS X (10.5.8)