There are 4 different sizes of Optiplex 780. With a 230W PSU you must have one of the SFF, or USFF sizes. The USFF has no expansion slots. The SFF needs a low profile card. You have a few options. Since you asked about the 1050 I will go into that first.
All the low profiles 1050s are 2 slots wide due to a hard mounted DVI slot, and also 2 slot wide cooling solutions. They won't fit unless you're willing to do some modding. The HDD tray is in the way. A laptop 2.5" HDD, or SSD can provide some room there. You will need to cut the back of the case for the DVI connector, and trim the GPU bracket also. I would suggest cut the case, and try the card w/o the bracket to see if it works for warranty reasons before modding the bracket.
The...