Power Supply Question... Relating to Video Cards...

I recently purchased a EVGA GTX 275 FTW Edition and must upgrade my power supply to use it. My card requires "550 Watt or greater power supply with a minimum of 40 amps on the 12 volt rail." Okay...I get the wattage and whatnot, simple enough. But what does the 40 amps mean and where can I find if a particular power supply has this? Every one I've looked at online (even $500 ones) have less than 30 at 12 volt... Can somebody explain this to me or atleast recommend an inexpensive power supply that will work with this card. Thanks!