Can a Dell Optiblex 755 handle GTX650 2gb

That computer comes in mini tower/desktop/small form factor/ultra small form factor designs. The mini tower is the one with the best chance to add a card, it allows full height cards to be used and it has a 300W power supply. The GTX 650 usually requires one 6 pin pci-ex power connector. I doubt your power supply has that, you can check though.

So you might need to look at cards that do not require such a connector, this mostly means used cards. Also, with a dual core and only 2gb system ram, you understand the types of games you can run are limited right? No GTA V, no Battlefield 1.