Many thanks for all of your replies. I was looking to do photo correction and video editing.
It sounds like you will do this as a hobby on a non professional basis. And for that I think you might be doing fine with what you have. However, you are very late in the game when it comes to the Mac Pro. I got my 5.1 about 5 years ago and thought I was late in the game. The 5.1 is already limited on the single core benchmark, the 3.1 is way below that. However, I am really doing fine till this day, so don't worry about this too much unless you expect wonders from it.
On the hobby and semi professional side of it, render time matters not so much. Regarding tuning, I guess its best to find the best balance between SSD and GPU tune up. There is not so much that you can do on the CPU side of it. I think the Quad 3,2 Ghz was the fastest you could plop in there, but most came with a 2,8 GHz.
On the SSD, I think its best to go for an older AHCI SSD blade that goes into a PCIe adapter card. Those are the Gen1 blades, a predecessor of todays NVME blade. Since you can not use NVME, you need to go with an AHCI blade and a good adapter. The best models to look for is the Samsung 951 AHCI with 512GB or a Kingston Predator blade (also AHCI) which is the 941 that came in 480 and 960GB sizes. I have both from the pre-NVME-Firmware era and they do work very reliable to this very day. However, it might take a while to source those on eBay depending where you live.
The AHCI standard still runs as SATA, but you have more speed compared to a regular 2,5 SATA SSD. Sadly most 3.1 owners don't know that and go for a regular SATA SSD.
On the GPU side of it, I think you want to have at least an Nvidia EVGA 680 GTX card in there. A 980 Ti would be a lot better, however both gpus need special flashed EFI Roms to work with boot screen if you plan on having windows 7 on bootcamp on there. The Nvidia Quadro K5000 might be worth a look as well if you can get it for a reasonable price. The 680 (EVGA) and the K5000 (PNY) have been available as Mac Editions back in the day, so chances are better to find a Mac-EFI rom for it to get it flashed. When it comes to Mac Pros in general, its all about the question which OSX you want to run on this machines. If you are fine with ElCapitan (which I would choose on a 3.1) you would be fine with an Nvidia GPU. If you want to run newer OSX Systems, you will need to have an AMD GPU version. The OSX actually is the deciding factor in this whole project because it determines which editing software will run on it. If you want to run Mojave or newer, I would recommend to sell the 3.1 and go for a 4.1 or 5.1 because of more OSX options and Metal capable AMD GPUs such as the 580. If an older OSX fits the bill, keep the 3.1, otherwise sell it. A good thing on the SSD tune up, - you could transition those to a 5.1 later on if necessary.
