A single 9800 GT video card uses 132 watts and 11 amps at full load.
Two 9800 GT video cards in SLI mode use 258 watts and 22 amps at full load.
Tom's Hardware recently published an article about video cards and actual power requirements. There is a link to the article on the home page. Most people asking about power supply requirements usually overestimate power requirements. It happens all the time at this forum.
doomsdaydave11 posted the correct answer. A high quality, efficient 550 watt power supply with sufficient amperage can handle two 9800GTX video cards in SLI mode.
invisik posted that he is running 2 gtx260 in sli with a quad core with a 500 watt power supply. I'll bet he is using a high quality psu.
Corsair, PC Power & Cooling, and Seasonic all have a solid reputation for high quality psu's that are very stable and reliable. They consistently earn high marks in technical reviews and comparisons. They are the brands most often recommended by veteran posters at this forum.
For example, the Corsair 550VX 550 watt power supply has a 12 volt rail rated at 41 amps:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139004
There are a lot of cheap low quality psu's with on the market. They are not worth the risk.