RX550 in 3040 SFF

That will not fit nor run on your SFF. Too big, too wide and needs a bigger power supply. The Optiplex will never be a good gaming computer. It is meant for basic office use. You need an older low profile GPU.

you need a single slot width low profile low TDP gpu. In this case gt 1030 low profile might be the only poor option, unless you want to install psu outside case (not great looking option) and install a double slot width low profile card GTX 1050ti or 1050.  the motherboard PCIe x16 slot is in proximity to psu preventing install of double slot gpu.

issues: gt 1030 does not have good performance/cost ratio. It is not a great gaming gpu.  gt1050/1050Ti recommended psu is 300W. stock SFF psu is 240W.  so gpu may be under powered which affects peak spec performance.  You can try to upgrade psu but again it needs to be outside case.

You can use Moddiy Adapter and put power supply OUTSIDE the case running the wires thru the hole left behind by the power supply removal.

Only short Cards like GT1030 or LOW profile GTX 1050 Ti

Non dell cards require Secure boot OFF.

https://www.moddiy.com/products/Dell-OptiPlex-3040-3050-3060-Main-Power-24-Pin-to-6-Pin-Adapter-Cable.html

https://www.amazon.com/MSI-GTX-1050-TI-4GT/dp/B01N2W8MJ9

https://www.youtube.com/watch?v=SCQTj1ei3Ew

Amen.

Please be seated.

Sister Mary will lead us in prayer. Afterwards, our special guess Robert Tilton will give a special sermon about cussing.

Then we will have the usual church bean supper.

Special music will follow, as always. Clothespins and febreeze spray are available in the lobby, just outside the sanctuary.

If I go with a PSU upgrade let's say 300 or over, is there any good compact GPU that would fit inside the Dell.

https://graphicscardhub.com/best-low-profile-graphics-card/

I have GT 1030 low profile graphics card in my dell optiplex and it's really good for what I want out of it.

fallout 4,Cs go,pes 2020 all low 1080p settings,plays pretty good.

8gb of ram and i5 3470 cpu!

Speedstep is back?  Whoa...