Good question.
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.
Good luck, and get an SSD first.