It looks like your computer has only a 300w power supply. If you get a powerful graphics card, you will need a heftier power supply. Make sure to get one from a good brand, like Antec, Enermax, Fortron Source, Corsair, Seasonic, PC Power and Cooling, etc.
If you shop online at Newegg the PCI-e cards often list recommended power supply requirements (though they are sometimes overstated). Make sure the power supply has the correct number and type of connectors for your choice of video card.