you see the slots on the black fan cage.
those are GPU supports and that means that little system can house a full sized card 10 inches
now that size is not an issue can you give us some details on the PSU?
assuming the slot will provide all the power look to a gt 1030 1050ti.
if the PSU can provide more power the RX 580s are just at budget, do not get a 4GB variant, pay for the 8GB RAM variants.
this is making huge assumptions, what is the CPU, an RX may bottleneck it to death.
knowing nothing else about the system I suggest a 1030 or 1050 variant. an improvement and powerwise not an issue but should play well with the monitor (the one the 710 is driving) and whatever CPU you have
**edit the r2 shipped with a choice of i7's 5th or6th gen? get the rx. get it now
power supply is ready for the rx