What's the budget?
Apparently, you can't have too much RAM for Photoshop, and video rendering and editing makes good use of hyper-threading. I think this puts you in the realm of a core i7 920 with 12 GBs of DDR3 (or 6 depending on budget). Or even a core i7 860 with 8 GBs of RAM.
Also, no gaming graphics but workstation??? I don't know much about workstation graphics cards, but here's the rest of the build:
Intel Core i7 920 Bloomfield - $288 or $229 if you're near a microcenter
RAM: G.SKILL 6GB (3 x 2GB) 240-Pin DDR3 SDRAM DDR3 1600
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231247
-$165
MOBO: ASUS P6T SE LGA 1366 Intel X58 ATX Intel Motherboard
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131386
-$185 after rebate
HDD for OS and most-accessed programs: OCZ Agility Series OCZSSD2-1AGT120G 2.5" 120GB SATA II MLC Internal Solid State Drive (SSD)
http://www.newegg.com/Product/Product.aspx?Item=N82E16820227462
-$339 after rebate
HDD for storage: Samsung Spinpoint F3 1 TB
http://www.newegg.com/Product/Product.aspx?Item=N82E16822152185
-$85
Case: Cooler Master 690
http://www.newegg.com/Product/Product.aspx?Item=N82E16811119137
-$80
CPU Cooler: Corsair H50
http://www.newegg.com/Product/Product.aspx?Item=N82E16835181010
-$78
If you run into budget problems, there are sufficient third-party coolers that can be had for around $30 like the Cooler Master Hyper 212. I'm imagining that you'll be doing some moderate overclocking. I think you'll definitely see the benefit in your rendering.
GPU: HD Radeon 4870
http://www.newegg.com/Product/Product.aspx?Item=N82E16814161292&cm_re=4870-_-14-161-292-_-Product
-$170
I figure you don't care much about Directx11 and games (lol, you own a mac currently), so this card should be fine at your resolution (what is it, by the way) and will actually handle some of the recent games fine.
PSU: Corsair 850 TX. May be a bit overkill for your system, but is only $110 after rebate and will still be more than 80% efficient at the load you put on it.
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139009
Finally, operating system: Windows 7 Professional
http://www.newegg.com/Product/Product.aspx?Item=N82E16832116758
-$139
You may be able to downgrade this to Home Premium if you don't think you'll be needing XP Mode or automatic back-up or other professional-specific features. That'd save you $30
That brings you to $1639 unless you get the microcenter deal, then it's more like $1589.
Hopefully I haven't misunderstood this thread. Are you planning to buy a Mac and "build" it off of their site? Building it yourself would be the more cost-effective way to go unless you desperately need some Mac-specific programs or something.
Good luck!