Here is the situation. An immediate family member has the following:
Dell Optiplex 790
CPU - i5-2400 3.4GHz
GPU - Nvidia 750 Ti 2GB
RAM - 16GB DDR3
It is doing OKish for what they are doing but a little extra performance would be welcome, and they have a free hand-me-down of the following;
HP Elitedesk 800 G1
CPU- i5-4690 3.7GHz
GPU - on-board only
RAM - 16GB DDR3
These are OEM systems so non-standard motherboard, PSU is non-standard, 320watts, no PCIe power connector, and CPU is throttled to around a little less (maybe 10%) than it would be on a Z97 motherboard. Both have SSDs and Windows licensed, and running on 1080p monitor. The user plays WoW and various other games, but they do not play fast-paced shooters like Overwatch.
I ran across this article which pretty much outlines some options. https://www.techspot.com/article/1841-gpu-cheap-oem-pc/
So we have a few options:
No cost option. Transfer the 750 Ti to the HP and call it done. Might get some improvment, but in many situations the CPU might be held back.
Buy a GPU that doesn’t require external power. The article gives a few options (GTX 1650, GTX 1050 Ti, Radeon RX 560), and tests them. Per the article the 1650 is probably a waste, while a 1050 Ti can be picked up pretty cheap used, like $80. Depending, it might be 50% better than the 750 Ti.
Buy a more powerful external powered GPU, various options in the $200-300 range. A PSU replacement would be required (which I have lying around), requires a $12 adapter for the non-standard motherboard power connections. I agree with the article that this is largely a waste.
Option 3, but instead use the 1080 GTX currently in my system and replace it with an RTX 2070 Super or better. Those are running around $500-550 currently.
I’m partial to options 1 and 4 here. Buying a new marginal GPU for $150-300 here is a waste of money IMO. Of course there is the choice of option 1, upgrading my GPU and building them a system around the 1080 GTX, but then the money involved here really starts to bloom.