Yes, you can upgrade it. The gpu only withdraws 48 walts and the power supply is around 180-250w (can't find the specific psu) but ether ways your fine. 
Hello, In the past few days i've been thinking about upgrading my PC for old games. I found a GeForce GTX 750ti for 42 USD on a popular polish online store. I want to put it in my Optiplex 780 MT for better performance. (the card doesn't have any 6pin connector) I also want to upgrade the ram, and i found out this PC uses two Samsung 2gb sticks. The PC have 4 RAM slots, so technically putting another two Samsung 2gb sticks would be fine. So can i upgrade it?   Links: GPU: [URL] RAM: [URL] (2x)   Note: I already upgraded the hard drive into an SSD
GTX 750ti on Dell Optiplex 780 MT