There is an issue with what you want to do. The current graphics card in your computer (HD5450) runs completely off the PCIe slot's power. The graphics card you intend to swap it for (GTX560) uses the slot power AND two PCIe power cables from the power supply - likely your PSU lacks those connectors. Also, a minimum of 500W is suggested for the PSU to provide the extra power needed.
What are your power supply's specifications? Make and model works if known, if not, on the side of the PSU is a label that says what the voltage and amperages are - I'll need that to answer you (just the +12v info)
Worst case scenario, you would need to swap out the power supply also