It really depends on what 450W unit it is, all power supplies are not created equal. 80% of a modern units power draw comes off the 12V rails so if its a 450W unit that can only give you 220W on the 12V rail you are going to have problems, if its a good 450W unit that can provide 400W from its 12V rail it would be sufficient so it really depends on which power supply it is, but for the most part very few 450W would be good for running a 460, you really should have gone for a 550-650W unit from the start
The GTX 460 draws ~160W full load so you'd need a PSU with about 30 amps on the 12V rail. Few 450W PSUs will provide this power @ 50C operating temp except top quality PSUs. A 550W-650W would be a better choice unless you buy a 450W top brand PSU like Seasonic, PC Power and Cooling, etc. that can provide the necessary 30 amps. on the 12v rail(s).