if you are having lots of crashing/freezing, it might be worth troubleshooting your system first, by testing one of your current video cards at a time, to see if the problem is with one of the cards and/or with cuda. if you cannot isolate the crashing/freezing to the video cards, a new one may not fix those problems...
for new video cards, you probably want one with a single 6 pin or 8 pin. you can use a cable adapter for (2) 6pin to (1) 8pin.
here are some cards, and power connectors. some power connectors vary by model, so you will want to double check on the card you purchase. the gtx 9xx series is being replaced by the 10xx series, so you might find some on sale or used bargains too.
gtx 960 4gb - 6 pin
gtx 970 - 8 pin
gtx 1070 - 8 pin
amd rx 480 - 6 pin
some of the new amd and nvidia cards aren't out just yet. the rx 480 is speculated to be slightly faster than the gtx 970 with a starting price of $200.
if you are worried about mac boot/bios, this website sells flashed cards. MacVidCards.com
several folks here just keep an old mac card around for emergencies and go without.
a gtx 960 and above will be several times faster than those old cards, so you should be able to buy one new one to replace the two you have now. some cards have blower style fans (like on your quadro) to help keep the heat out of the case as well. if you have any programs that require cuda, you will want to stick with the nvidia cards.
if you want/need quadro, the quadro m4000 (6 pin) performs somewhere between the gtx 960 and 970 in speed. the new M2000 is somewhat slower than the gtx 960. the M2000 appears to have no power plugin...
I know very little about Macs, but, I can see a couple of items of concern. First, the Quadro card has only about 1.8 GB of video memory and although it is above the minimum threshold required for CUDA, ( 1 GB minimum), it is even less than what is on my laptop and would be weak with 4K. I have read that the GeForce cards offer far better performance for cheaper......maybe getting rid of the ATI card and ADDING a good GeForce card to the quadro could be a solution. The two cards can operate together, I believe......others like Eric Bowen and Bill Gehrke here on this forum would be able to advise better than me....its worth checking out. Generally, it is better to have at least 4GB of DDR5 video memory on your NVidia GPU, especially when trying to do 4K editing.
Next, you have not described your drive system. This is the MOST common area that causes bottlenecks, freezing, and stuttering due to inadquate speed of the drives. Assuming your operating system is fully updated, you would want to make sure your drive configuration is adequate.
Currently, because SSDs are so cheap, most people have their OS and programs ONLY on the "boot drive", which is a quality SATA III SSD, like a Samsung 850 Pro, or, the Crucial MX200. Then, all other files can be placed on a larger SSD, a RAID 0 of several SSDs, or, onto a new, incredibly fast PCI SSD, like the Samsung 950 Pro for best performance. Toshiba has just come out with a 1TB PCI NVMe SSD for just over $700 which has been reviewed at storagereview.com. The Samsung NVMe 950 Pro is 500GB at around $350....either of these two drives would provide incredible speed of over 2GB/sec read and 1.5GB/sec write, compared to the stone age speed of a single,spinning 7200rpm hard drive going at 100MB/sec read and write.
If you are using single, spinning hard drives, it would be time to upgrade your drive system.
Pull the ATI card and try that. If you want a new GPU then a 970GTX would be about all that system can push. I would check MACVIDCARDs and see what they have flashed.
way to sum up my very long post in few sentences, lol.