Looking for a Mac Pro

[kohlson]

For example this iMac might be a good fit

Right - a late-2103 with dual core 2.9 GHz processor and GT750M GPU. This is nearly the same as my late 2013 15-MBP, which has a 2.6 GHz i7. Certainly the newer iMac will be a better fit for FCPX.
I use FCPX to create short vids (3-5 min) and have expanded into short documentaries (10-20 minutes). For the latter, these projects are typically 60-100GB. Note that with background rendering, they fluff right up to 300+ GBs. You're going to need some disk space! All of my work so far has been in HD, and it works OK-there's even a 4K monitor connected.

It's pretty easy for me to see that this will be strained for 4K. That's why I've ordered an RX 580 8GB for the 2009 4/5,1 MP sitting right next to it. It's a 3.33 6-core, with 32GB RAM. A 256 GB SSD, plus a few TBs of HDD. Also, an Inatek KT 4004 for USB3. By many accounts, this set up should work for 4K.

 

[pl1984]

Right - a late-2103 with dual core 2.9 GHz processor and GT750M GPU. This is nearly the same as my late 2013 15-MBP, which has a 2.6 GHz i7. Certainly the newer iMac will be a better fit for FCPX.
I use FCPX to create short vids (3-5 min) and have expanded into short documentaries (10-20 minutes). For the latter, these projects are typically 60-100GB. Note that with background rendering, they fluff right up to 300+ GBs. You're going to need some disk space! All of my work so far has been in HD, and it works OK-there's even a 4K monitor connected.

It's pretty easy for me to see that this will be strained for 4K. That's why I've ordered an RX 580 8GB for the 2009 4/5,1 MP sitting right next to it. It's a 3.33 6-core, with 32GB RAM. A 256 GB SSD, plus a few TBs of HDD. Also, an Inatek KT 4004 for USB3. By many accounts, this set up should work for 4K.

Perhaps the OP can give us additional information on what type of video work he's performing. Until then I am going to assume that his 2008 era iMac was adequate for the work he does. Therefore replacing it with a similar, more updated and capable model, should not be a problem. At least not until Apple releases the new Mac Pro at which time the OP can re-evaluate his options at that time.

As for the RX580 keep in mind you already own the cMP, you are not laying out the money to buy one in addition to the RX580. If the OP already owned a cMP then I'd be offering different recommendations.