GTX750Ti is the go to card for these. Unless you have the Mini Tower you need a low profile single slot card. You will need a card that is not UEFI rated. Also known as Legacy BIOS. GTX1050 w/o 6 pin PCIe should also work. You can run bigger GPUs in the Mini tower with a PSU swap.
It's a BTX. If you get a full height (1 fan)card it will probably have 2 slot bracket. Get one w/o connectors on the 2nd side. remove the bracket and teat the card. Cut the bracket in 1/2 and install the card. Put the other 1/2 behind the card to let hot air out. If the card has a full length cover you will need to trim away the back 1/2" so air can go out the rear vent. If you have high GPU temps. an expansion slot blower will be needed.
Many low profile...