Re: PSU for 670
Your 500 watt PSU should be enough. A 670/660 Ti only consumes about the same amount of power as a 460 so you should be OK.
Personally I would go with the 670 over the 660Ti, it depends on the resolution of your monitor and if you like to play with high levels of anti-aliasing though. If you play at 1920x1080 and don't mind using FXAA as opposed to traditional MSAA, the 660Ti is probably a better value though.