Okay, looks like you have one PCIe x16 slot on your mobo and you have a stock ATX 300W PSU. Without replacing the GPU I would not go with anything more than a 75W GPU, a good match would be a GTX 1050 Ti. This will be a pretty even match for the CPU.
If you wanted a stronger GPU you would have to replace the PSU and then you could go for a RX 470 or a GTX 1060-3GB. These will over-power the CPU a bit but should still do. Anything more will just bottleneck.
Here are some potential GPU's to review
https://au.pcpartpicker.com/products/video-card/#c=380,378,376&sort=a8&page=1
Not sure how old your PSU is but it might be consideration to replace it regardless. Cheap PSU's will have a lifespan of around 5 yrs, I see this desktop was released in 2012.
Tier Two (Better) PSU's
https://au.pcpartpicker.com/products/compare/Q36BD3,2tckcf/
Tier Three (Good) PSU's
https://au.pcpartpicker.com/products/compare/Yf7CmG,XCjG3C,wnQRsY/