His radeon 4670 iceQ on a 300watt power supply?

Which computer is it?

300W will be pushing it with that card. The manufacture states 400W for a reason. You will want to upgrade your PSU to fit the recommended specs before installing that card. Sometimes you can get away with a lower wattage PSU if it is of good quality and can handle the current on the 12V rails. I'd say maybe a SOLID 350W, but I have a feeling your 300W is an OEM style.

It really does depend on what you've got in the rest of your system. One of my machines is running an HD4650 on a 350W PSU with no problems at all, even though the manufacturer specs for that card say 400W. Manufacturer specs are generally going to err on the high side because they're just guessing at the setup for hte rest of your machine; you could theoretically have anything.

On the other hand, 300W is not a whole lot to work with. The 4670 doesn't use a whole lot of power, but like others have said, it's impossible to know without the rest of your system specs. I'd be surprised if it checks in at over 300W total, though.