optiplex 990

Hi! Recently, I bought an Optiplex 990 non-sff (i5-2400). I've been having some issues with GPU's, as my r7 260X isn't working in it (the PCIE slot is good, I have a good PSU - 600w - and the GPU is good), but my HD 6570 works. So, does anybody have any suggestions for or know any GPUs that...

(I don't know if this has been covered exactly yet, as others' situations don't seem to match mine. Also, didn't know if I should post this under GPU or motherboards, so I just put this under GPU) So, recently, I purchased an Optiplex 990 (not sff). It came with a 500w PSU, so I thought it would...