Lightroom is constricted by the CPU, not the GPU. The best upgrade in this department would give you a boost, but not a "night and day" increase. That would be the i7-3770k. You should easily be able to find this on Ebay for under $200.
As for Photoshop, your GPU should be enough. Where you may run into problems with the GPU is the amount (2gb) of onboard RAM. Again, it shouldn't be an issue, but if you are running multiple monitors or working with extremely large files, you should consider an upgrade to a GTX 1060 6GB.
Now, all of this being said, your system specs should easily be able to push both Lightroom and Photoshop with ease. The most important recommendation is left for last - USE AN SSD! Format fresh on your new SSD and load everything from scratch. I don't know if the 3X drives are in RAID config., or if used for storage. Either way, if you can only afford one upgrade - put your OS/programs on a Samsung SSD. When you are working, transfer everything you need to the SSD. When done, dump it to the older spinners.
Ideally, you will get rid of all your 7200 HDD and upgrade everything to SSD, but that can be expensive.
I'll follow the herd on this one, get an SSD, that's going to give you more mileage on that build than anything else. A nice 850 evo won't set you back and will outlast the rest of the system. At some point you'll want to do a full refresh but you can likely get another year or two out of that build, minimum. Don't spend a dime on another core component after the SSD, if anything fails other than that PSU or the platter drives (which can carry over), do a full refresh.
If you get Samsung, you only need to "format fresh" if your Windows is infected with bad stuff. If not, their free software can migrate Windows to the SSD if you hook it up with either an internal SATA cable or an external USB-to-SATA adapter.
That said, Coffee Lake releases this week, and the i7 8700 will offer 4.6 GHz turbo at stock speed vs. 3.8 GHz for the 2600, and both the instructions per clock and turbo are improved. It's also 65 watt TDP instead of 95 watt.
If your budget allows it you'd see a very nice boost from keeping the graphics card but switching to: i7 8700, cheap Z370 motherboard, 16 GB DDR4, SSD.