You can run Snow Leopard on the new macs, so I'd do that rather than running Lion - which goes to show how long it was since Apple refreshed their Mac Pro hardware.

The 8 Core 2.4Ghz seems to be the best compromise between performance and price. RAM makes no odds for Catalyst, so the standard 6Gb is fine.

You can buy 3.5" SSD's - OCZ's Colossus range for instance, so you don't need to buy 2.5" to 3.5" adapters.

Avoid the standard issue ATI graphics cards. They give dire performance.
You have a few choices when it comes to graphics cards:
PNY's Quadro FX4800 for Mac
nVidia Quadro 4000 for Mac

I have had some success with re-flashed GTX285 graphics cards, as have others.
The EVGA GTX285 was the graphics card of choice (and still is) for Catalyst by it was discontinued a few years ago. However, it is possible to get re-flashed PC GTX285's on eBay. These have been re-flashed to work with Mac OpenFirmware. Having built, sold, installed these into several rental and installation Catalysts, I have yet to have one go wrong. They are certainly a cheaper option to the Quadro cards and have two DVI outputs.

Best wishes,

Hugh