It doesn't matter as long as the total wattage on the 12v supports the graphics card or cards that you want to use. The 12v power is the MOST important thing to consider (other than reliability) when buying a PSU, even more important than the PSU's total wattage.
When a card lists "minumum x watts" it doesn't mean the PSU's total wattage. It's referring to the wattage of the +12v rail specifically.
If you get a PSU with 2 12v rails and neither one has enough wattage on it's own, you're boned unless the card has 2 6 pin connectors.
Ditto if you have a single 12v rail that doesn't have enough total wattage for your (hypothetical) SLI'd/xfire'd set of cards.