|"how do you know that I won't need to upgrade the power supply for the Radeon HD4670"|
Did you read the article?
"its main advantage is a low-power requirement—the 4670 doesn’t need an external power cable, which for some upgraders is a really important feature."
"I tried looking for a wattage, but I didnt see anything."
From another Tom's Hardware article about the HD4670:
"With a maximum power draw of only 55 W, the HD 4670 draws less power than many modern CPUs, which explains why no auxiliary power connectors are needed for this device"
"as a way to inject new life and capability into a PC that’s more than a year old, the HD 4670 qualifies as a "best technology buy" for such systems. About the only thing that might do more would be to bump up memory to 2 GB for Windows XP and 4 GB for Vista systems."