I have an Apple Cinema Display 30" and use it with dual-link DVI and a couple of Hacs and a macpro3,1 for years. One of the lamps or bulbs was dimming and I moved it out of the way and put an LG Ultrawide on my desk instead.
FWIW I ran the ACD 30" in HiDPI mode on every Mac I ever attached it to using SwitchResX.
I bought the GTX 680, It does 4K, or some does ( It was a flashed or mac version all ports working ).
Only you need another screen because 4K is black until you login.
It was a 43" IPS, so much workspace ... never tested ultrawides.
I had a vertical 24" next to it, the hight is the same as the 4K.
A 27" or 2 or 2560x1600 would also work.
But never looked back once I went 6,1.
I was surprised by the trashcan.
NVME or AHCI somewhere thunderbolt, 3m away.
Same 4 my 4 disk enclosure with 4TB spinners.
And I have a lot of docking stations to swap Sata SSD's like floppy disks.
2560x1600 is a lot of the time way more expensive,
than 4k or 1440p.
I still love my 23" original cinema display.
It's has 1200pix, a lot more comfortable than you would suggest.
But mostly a lot more pricey, same as the 30".
For the same money I think you can get a gtx 680 and 4K or widescreen.
(would not crave a 4k that's somewhere in the 20", don't know if scaling would work well )
I also ran my 3,1 with a GTX 680 and it was reliable. Way more reliable than my 6,1; hell since I didn't move to BigSur I think that 3,1 was probably at about a 6mo uptime from when I added disks to the zpool, the only reason I pulled the 3,1 was that it idled at 40% and couldn't run Docker because that generation of Xeon is missing some of the memory and virtualization flags.