You could get a 6670, the fastest GPU currently on the market that does not extra power from 6/8-pin PCIe power cables. As vmem suggested, a 7770 should also be well within a 450W PSU's capabilities if it has at least one 6-pin PCIe cable.
My 2 cents here. Have a 5770 on a 300 watt power supply(mind you the parts had been picked for low power consumption as well) without issues. The 7770 takes less power so unless it is a real bad no name 450 watt psu(you have to watch for those 450-500 watt units with under 250 watts on the 12 volt rail....), it should work