Can a 420w max PSU support ATI Radeon HD 5670 Graphic card?

There are several other things to know about your PC before stating that this is the best PSU for your system. Is this the only video card? How many other drives (hard and optical) do you have installed? What other devices, if any are connected to the board? Without knowing all the specs it's hard to say but since this particular card doesn't connect directly to the power supply itself - it will in all likelihood work with the unit you have installed. HOwever to get the best life range and performance all the above questions really need to be addressed.