Don't compare the X1800XL to the 7600GT. Look at either the X1800GTO or the 7600GT, or the X1800XL and the 7800GT (which both make no sense to buy now for reasons).
I can't locate the voltage information for those specific cards at the moment, however I think that for an X1800 series card your going to want at least 24-26A. Also, I'd reccomend an Antec or FS Nortron PSU, as that brand isn't really reliable. You'll also want that extra power and ampage for OCing and the ability to upgrade in the future.