Here is a link to a Tom's Hardware article showing total power consumption as measured at the electrical wall outlet (mains) for a pc system with one 4890 video card installed:http://www.tomshardware.com/reviews/radeon-hd-4890,2262-13.html
Here is a link to the xbitlabs.com technical review:http://www.xbitlabs.com/articles/video/display/radeon-hd4890_6.html#sect0
Finally here's a link to the anandtech.com technical review:http://www.anandtech.com/video/showdoc.aspx?i=3539&p=22
The 4890 video card is not a power hungry card. In theory you should be okay providing there is sufficient current (amps) on the 12 volt rail(s). However, I could not find a technical review of the power supply. In addition, it appears as if the power supply was first introduced in 2005 and that it is not 80+ certified.
The general rule of thumb is that a high quality 500 to 550 watt power supply with sufficient current (amps) on the 12 volt rail(s) can easily power a system with any single video card made.
Brands such as Corsair, PC Power & Cooling, Seasonic, and a few others have a reputation for high quality power supplies. They consistently earn high marks in technical reviews. They are very stable, reliable, and come with 5 year warranties.