Wow, you have described my machine almost to the last detail. I have a mid 2010 Mac Pro, 24gb and the radeon HD 5770 card and an NEC wide gamut monitor. I pretty much used the information I gleaned from a website, http://macperformanceguide.com
Based on your description of what you are considering, I get the feeling you've already been to that site. I keep my apps and boot off a small SSD kept in the #2 optical bay. I have a 3 drive Raid 0 stripe that serves as my data drive. The 4th drive is partitioned into a boot backup and a Time Machine drive. I have an external eSata drive enclosure (multiple drives) that does backups daily of my data partition. I use SoftRAID 4.3 to manage the Raid.
Running your apps from an SSD drive makes them wonderfully fast. Keeping data on a 3 drive HD Raid 0 stripe makes your data access very fast as well, no need to go with all SSD's until prices come down some more. I regularly run LR4, PS6, all the Nik Plugins, Acrobat X, InDesign6, and the MS Office Suite. Everything is fast and snappy.
I can't answer "how much faster" it would be than the four core but I believe that is addressed on the website I mentioned previously. My understanding re:upgrading the video card is that it is not a noticeable improvement for still photography editing.
After 2 years with this machine, I want for nothing (well, at least pertaining to the workstation). It is the first machine I bought without trying to rationalize why I could get by with less and I have not regretted it a single day.
I have a six year old Mac Pro that I am looking at replacing soon. I use it for Photoshop CS6 and Lightroom plus various plugins and accessory software like Nik Color Efex, Topaz deNoise, etc. I often work with large files (500MB to 1500MB). I plan to install either 16 or 24 RAM. How much faster would the 3.33 hz six core ($500 extra) be than the 3.2 hz four core? Same question for upgrading from the Radion HD 5770 1GB to the HD 5870 graphics card ($200 extra)? I would like it to be sufficient for running Photoshop for another five or six years.