the gddr5 version of the gt240 is the fastest out of them. ddr3 5570 has similar performance to a ddr3 gt240, same thing with the ddr5 5570 having similar performance to the ddr5 gt240. i guess this image shows it, although it doesnt show the gt240 ddr3.
i use quick edit. well gddr5 is twice as fast as ddr3. it makes hardly any difference in these low end cards whether you buy the 512mb or the 1gb version but there is a difference between ddr5 and ddr3 as you can see in that benchmark i posted.
the HD5000 series can use opengl 4.1 just like the gt430. i think theres opencl support aswell now with the HD5000 series. the gt240 and HD4650 use opengl 3.3. not sure about the performance in opengl but they do use different versions.