I agree in general that an RX 560 or RX 570 might offer the best value around this price range if their system is compatible with one, but you also need to consider the power supply necessary to run them. There are versions of the RX 560 that can run without an external power cable, but the RX 570 would likely require a better PSU than a Dell Optiplex typically provides. What kind of power supply does the system have?
Also, if it's in a small form factor case (a thin one), you would need a half-height graphics card to fit in there, which you won't find with most RX 560s or any RX 570s. Optiplex 990s come in a vaiety of shapes. What shape is the computer's case, compared to the one's shown in the image on this page?
https://www.dell.com/support/article/us/en/19/sln285254/dell-optiplex-990-system-guide?lang=en
Also, the closest competition to the RX 560 in terms of performance is actually the GTX 1050. The GT 1030 performs more similar to an RX 550, well behind those other two. There are two versions of the RX 560 though, a 1024 core version that on average performs fairly similar to a GTX 1050, and another 896 core version that performs about 10% behind in most games. And the RX 570 actually performs most comparable to a GTX 1060 3GB, far above the performance level of a 560, 1050 or even 1050 Ti, but again, requiring a decent power supply and a standard size case that can fit it.
Edit: And what is your budget in your local currency? I notice pricing of graphics cards at that site might not directly follow pricing in USD going by the exchange rate alone.