Is your Optiplex 9020 the MT version or SFF? If you aren't sure. which computer does it look like from this
photo (left or middle).
If it is the SFF version, the best GPU you'd be able to get is the GT 1030, as the PCIe x16 slot is power limited to 35w. If it the MT version the PCIe x16 slot supports the standard 75w, allowing for a GPU as high tier as GTX 1050ti. If that is the case the 750ti would also work.
Looking at your PSU wattage, it seems to be the MT version. It seem to be compatible, just make sure the GPU you get is
below 16.7cm.