rockie_ :
R7 250 has 384 shaders
GT 640 - 384 shders
the core clocks are close but GT 640 has DDR5 VRAM , so
i would go for the GT 640
You can't compare shader counts on GPUs with different architectures, a shader on Kepler may do more or less work than a shader on a GCN based card.
Having said that, the R7 250 does bench higher than the GT 640, but that's the DDR5 version, the DDR3 version will be a bit slower. DDR5 RAM would help the GT 640 a bit, but probably not by that much, it's not a particularly fast GPU to begin with. The DDR5 version of the GT 640 and the DDR3 version of the R7 250 would probably trade blows between each other in this case. The best option would be to get a DDR5 version of the R7 250 which is indisputably faster than any GT 640.