No. If your PSU is a cheap one that came with your case, its label will be a lie and it won't be able to provide 450W; 300W-350W is more likely, which is not enough for a GTX560Ti. The most powerful card you can safely run on your PSU is a HD7750. If the PSU has a PCIE power connector, you may be able to run a HD6770, but the difference between "may be able to" and "will be able to" might be measured in sparks, smoke, and a dead system.
Instead of buying a GTX560Ti, I would suggest buying a quality 500W PSU (such as Seasonic, Antec, or Corsair), which should leave you enough money to get a HD6870 or GTX560 (non-Ti).