I almost couldn't believe my eyes.  I checked the 755 Tech Guide and you have a 275w PSU.  Either one of those cards will work, so will the GTX 1050 ti.

So consider your CPU and potential bottleneck.  Your CPU isn't listed in your first post, so here's an example of how to check:  In the Google search bar, "Q9650 GTX1650 bottleneck," Q9650 being an example of a CPU.  Bottleneck would be 68.17%.  That's some serious bottleneck.  Anything below 10% is good.  PC-Builds   The GT 1030 and GTX 1050 ti are probably better bets.

If you don't have it already, Q9650 is the top CPU for the 755.

I think you need to be careful installing gpu in 755 SFF, which uses BTX format mother board in a tight case. I doubt you can install a double slot width low profile card in it. 
the hdd is in the way. gpu has to be very short too, or the cpu heatsink gets in the way too.
https://cdn11.bigcommerce.com/s-a1x7hg2jgk/images/stencil/1280x1280/products/13569/77914/dell-optiplex-755-sff-intel-core-2-duo-2.33ghz-2gb-ram-80gb-hdd-8.39__85336.1490082477.jpg

As you’ve said the GTX 1650 has a 60% bottleneck with the highest possible CPU, what GPU would you recommend that doesn’t have such a high bottleneck percentage?

Thanks for helping

@redxps630,

Would you recommend the GTX 650 ti BOOST you recommended in the 780 SFF thread?

Here's a video with a GTX 750 ti and a 755 SFF, but it doesn't show installation.  https://youtu.be/Im1ANseATF8 

I just reviewed a video in which the HDD cage had to be removed for something like a GTX 1050 ti to fit.

I can't guarantee it, but it's short, single bracket, and might fit:  GTX 750 ti eBay 

Bottleneck with E8400 = 11.94%.  2.71% with the Q9650 according to PC-builds.

YouTube List of your chassis type running the GTX 750 ti.

This generation of Optiplex's - 7010, 7020, 9010, & 9020 MT versions does give you more options.  Not only a little more leeway with GPU's, but PSU's are also upgradeable further expanding the possibilities.

Re: As you’ve said the GTX 1650 has a 60% bottleneck with the highest possible CPU, what GPU would you recommend that doesn’t have such a high bottleneck percentage?

GTX 750Ti and Q9650 will work great together. 750ti is a 60w card that needs 300W psu.

Intel Core2 Quad Q9650 (Clock speed at 100%) with NVIDIA GeForce GTX 750 Ti(Clock speed at 100%) x1 will produce only 2.71% of bottleneck.

GTX 650Ti boost works well w Q9650 too, but it requires a 6 pin power connector and 400W psu.

In contrast to: Your processor is too weak for this graphic card.

Intel Core2 Quad Q9650 (Clock speed at 100%) with NVIDIA GeForce GTX 1650(Clock speed at 100%) x1 will produce 68.17% of bottleneck. Everything over 10% is considered as bottleneck.

low profile MSI GTX 750 Ti in a desktop Optiplex 760 DT.  
https://m.youtube.com/watch?v=WR83S2gYutM

for SFF this could work too, but hdd cage has to be removed and hdd/ssd installed in 3.5” drive bay underneath odd.  but if hdd cage fan is disconnected from motherboard it will likely incur a bios startup error “failed hard drive fan.”

I have Optiplex 755 DT and i want to upgrade it to a gaming pc .

I want to buy Q9650 and GTX 1050 low profile. 

Will they work on the computer or not ??

@TomboMumbo, I edited my post above.

755 technical guide did not include Q9650, but Dell has validated Q9650 in this model. 

CPU Upgrade for Optiplex 755 - Core 2 Quad? These are the Quads we validated -

1050 would be significantly bottlenecked by Q9650.

Your processor is too weak for this graphic card.

Intel Core2 Quad Q9650 (Clock speed at 100%) with NVIDIA GeForce GTX 1050(Clock speed at 100%) x1 will produce 19.96% of bottleneck. Everything over 10% is considered as bottleneck.