Anything lower spec than hd 58xx is pretty pointless and the HD 5xxx and 6xxx series are pretty much only good for music coin, EXP or cryptonight.
I don't have any 5xxx cards still mining but I do have a couple of dinosaur rigs of HD 6990s they're horrid on power consumption though but can still make a tiny profit on cryptonight and EXP.
Just being curious, what could a HD7750 mine in your opinion ? I've removed it from my PC because for the moment I can mine with only 2 GPUs.