If your PSU is indeed 375W, then the most powerful one your computer can handle will be the AMD HD 7750. Open the side panel of your computer and you should see the sticker of the PSU with the information, take a picture and post it here if you do not understand the technical information of your PSU.
The PSU looks like this for example. It's the metal box looking thinging with all the wires coming out of it. The sticker is usually located on the side, it has all the info including make, model and watts along with amps and such.