Hi,
The GPU requirements are mainly RAM and PSU. The R7 250x requires a PSU of 450W (minimum) So a recommended one would be 500-600W. If it is a 450W then make sure the quality is good. The 250x requires minimum of 1GB RAM and 2GB recommended. So RAM should be OK.
Please not that the r7 250x was a entry level card.
I would strongly suggest to invest in a better CPU now (because 250x compares to the integrated GPUs) which has integrated GPU (eg. i5 or the APU range) and later invest in a discrete GPU.
Also the APU can only get you so far on games. But its good for medium settings on medium resolutions. You can also Crossfire later for improved performance.