For CS6 and FCPX, you would want to maximize the processor, RAM, hard drive speed and graphics processor, more or less in that order. That said, the i5 processor and 8GB RAM that come with the 27" iMacs is plenty for CS6 and FCPX.
If you want more RAM, it will cost less to order from Crucial or OWC instead of Apple. And the Crucial or OWC RAM will work perfectly.
Beyond that, I would strongly encourage you to invest in 1 or more external FW or Thunderbolt hard drives for backup and potentially additional online storage before even thinking about investing in a Fusion drive or the top end gpu w/2GB VRAM.
I don't have CS6...
1) I would max out the video card. FCP X and Motion 5 uses the GPU over the CPU for processing power. Once you buy your new iMac you can't upgrade this...
2) FCP X and Motion 5 will use all the ram you can throw at it. I have 16 GB of ram and both programs at times uses all 16 GB of ram. I really should bump up my ram to 32 GB. I would say 16 GB is the minimum for video.
3) The processor would be last on my list. Computers are really fast even with an i5 processor. Once you buy your new iMac you can't upgrade this...
This is the iMac I bought last year:
27" iMc mid-2011
3.4GHz Quad-Core Intel Core i7
16GB 1333MHz DDR3 SDRAM - 4x4GB
2TB Serial ATA Drive + 256GB Solid State Drive
AMD Radeon HD 6970M 2GB GDDR5
Apple Magic Mouse + Magic Trackpad
Apple Wireless Keyboard (English) & User's Guide
Thanks for the replies.
I've heard as well that the new nvidea cards in the imac will be used more than the radeons for the adobe CS6 programs. If the i5 is powerful enough to run Final Cut smoothly then I will probably look into upgrading the video card.
No one really said much about the Fusion drive which has me curious as it seemed a big deal at the Apple Event. Would it not affect FCPX or CS6 to have that extra 128gb of SSD? Or for graphics and video would I be just fine with standard HDD that it comes with?
The Fusion drive is a nice idea. But it's very, very new, and a 128GB SSD is not large enough to make a difference in your use of CS6 or FCPX, which will require significant amounts of disk space in operation. Video, in particular can easily require significantly more disk space for swap space, editing & rendering ops that you don't even see being used, but it is.
For the $, I would invest in a larger 7200rpm hard drive and an external thunderbolt or firewire drive for backup before worrying about investing in an SSD or Fusion drive.