The PSU in this computer is a proprietary shape and finding an aftermarket replacement is not easy. In fact this is the only one I know of, truth be told that I suspect the build quality, only about 250W on the 12V rail.
https://www.atxpowersupplies.com/350-Watt-Dell-F5114-Power-Supply-F5114-350W.php
Quite honestly, do not get this GPU, it will not work for a couple of reasons. It is a 190W card, you would likely blow up the the replacement PSU above if you installed this card. Definitely would smoke your 280W PSU.
https://www.techpowerup.com/gpudb/244/radeon-hd-4890
Also, pretty sure that the PCI-e x16 slot on this mobo is a restricted wattage slot, will only supply 25W and not the normal 75W. You can see it stenciled right onto the board (see pic in the link). This presents another problem in the you have two 6-pin cables from the PSU, each supplying 75W max. So the total power that can be provided is 175W (75W+75W+25W = 175W). This also presents a problem in finding low power cards that have 6-pin PCIe connectors.
https://sc01.alicdn.com/kf/HTB1bLZjIXXXXXXsXFXXq6xXFXXXo/221231019/HTB1bLZjIXXXXXXsXFXXq6xXFXXXo.jpg
This means the following for TDP wattages
No 6-Pin connector on card: Wattage range up to 25W
One 6-Pin connector on card: Wattage range up to 100W
Two 6-Pin connectors on card: Wattage range up 175W (but as stated before I would not be using these because of PSU)
Next the 755 DT only takes low profile cards, this likely excludes the HD 4890
To top it all off, the HD 4890 card is really old and isn't very powerful.
So your problem comes down to finding a low profile card with at least one 6-pin power connector that has the same or better performance as compared to the HD 4890 and which will be compatible with the PSU, motherboard and BIOS. Here's a database that shows the different chipsets that have one 6-pin connector.
http://
You can also set it for two 6-pins.
I tried to find a solution using the connectors filter in TechPowerUP, the TDP filter in PCPartPicker and the low profile filter at Newegg. No luck. Maybe there is something out there, I don't know.