What GPU (if any) do you have now? Is this a pre-built (Dell, HP etc)? If so, which model?
And what are you looking to achieve? Gaming? If so, what titles?
What kind of budget would you deem "low"?
Anything modern should compatible (GT1030, GTX1050, RX550 etc), although the pre-built's can sometimes have limitations from a BIOS front.