It is difficult for two reasons. 780SFF psu is mini form factor (smaller than TFX) and has mini ATX 24 pin. 235W Watt PW116 H235P-00 Desktop Power Supply Compatible with Optiplex 760 780 960 980 990 SFF Item Dimensions: 8.27 x 3.23 x 2.44 inches(210 x 82 x 62 mm) Connectors: P1 - 24-Pin Mini ATX Power P2 - 4-Pin ATX Power P3 - SATA Power P6 - Slim Line SATA The only 300W psu I can find is FSP300-60GHT 300 Watt TFX 3 3/8" Wide X 2.5" Tall X 6 7/8" Deep One ATX Connector (24 pin) One P4 Connector Two Molex Connectors Two SATA Connectors One Floppy Connector It is bigger than 780 psu, which means may not fit inside 780 case It has 24 pin ATX, which means you need to find adapter to convert into mini ATX 24 pin.