In theory it could help, but there are many other variables that can come into play.
What is the old GPU that it is replacing? What OS are you using? Will the drivers be compatible with your system?
It is difficult to predict how your performance will be affected by upgrading until you actually try it with your system.
The system GPU requirements are here: https://helpx.adobe.com/lightroom/kb/lightroom-gpu-faq.html
Would this GPU make things faster? We don't know becuase we'd need to know the exact functions in Lightroom that are slow. GPU will only speed up some tasks, and not others (it may actually slow down some tasks). Also please state your version NUMBER of Lightroom, and the size in pixels of your monitor.
My answers do not cover Photoshop, only Lightroom.
Photoshop and Lightroom both take the advantage of GPU's different. For Photoshop its's more likely that the GPU will be compatible. For Lightroom take a look at this article Adobe Lightroom GPU Troubleshooting and FAQ and GPU notes for Lightroom CC (2015) for recent developments.
Many thanks for all your quick replies!
My current configuration is:
Windows 8.1 Professional
Lightroom CC & Photoshop CC 2017 (I have a CC Photography plan)
Asus Z87-A Motherboard with integrated graphics card, Intel HD Graphics
Intel® Core™ i7-4770, 3.40GHz
Samsung T220 display, 1680 x 1050 resolution (I am planning to upgrade to a BenQ IPS display soon).
I checked the lists you mentioned regarding the recommended Adobe options and I see that the 900 series is there.
I am not a gamer, so I never thought I would need to get a crazy expensive GPU, but I started taking photography really seriously last year, so I have to upgrade. I use mostly LR. PS I use just for minor tweaks (but who knows, I'm just starting to learn). For example, trying the focus stacking technique for a macro shot, my system was blocked for about 5-7 minutes and I only had 5 images stacked.
Navigating in LR and applying various changes also works a bit slow.
Focus stacking is a Photoshop issue, and I can't comment on that.
We need to know the exact steps in Lightroom that are slow in order to know if you will benefit from the GPU in Lightroom. In particular, we also need to know if you are performing a lot of brushing and/or a lot of spot healing in Lightroom. Saying "Navigating" and "applying various changes" is not specific enough (and "Navigating" in what module?)
Also, since your monitor is 1680x1050, most likely a GPU will not help with this monitor in Lightroom. I notice you did not state the dimensions of your planned BenQ IPS monitor, that would also be needed to determine if the GPU might be of some assistance when you have that monitor.
I plan to buy the BenQ BL2420PT, which is a 23.8-inch display with a 2560 x 1440 resolution.
I work with files that average at 20MB in size and navigating means navigating between photos in the Library and Develop modules, it looks like it takes LR some time to load each file. When I say "changes" I mean applying presets to images and copying settings between images. Same goes when working with adjustment brushes and filters (radial and graduated). It works fine only when I change an individual setting, like exposure or contrast.
Exporting the files to JPEG also takes a long time, if I have about 500 files I have to leave the export to work over night.
So, scrolling from one photo to another will not be sped up by adding a more powerful GPU.
If it takes a while to apply changes like the adjustment brush, this is another area of Lightroom that is not sped up by a more powerful GPU (in fact, using a GPU slows down brushing).
The solution to both of these is a more powerful CPU (which will also speed up your exporting).