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!

Excellent recommendation and explanation.

And a special tip of the hat to OCZ for including a picture of the label showing the power chart in their sale listing. Every PSU should have that.