I've heard Dell uses good Power Supplies and recommended wattages for cards are often overrated because of the poor power supplies that a lot of pre-built computers use.
Here's one calculator for you to check. There are many other calculators on the net as well.
http://educations.newegg.com/tool/psucalc/index.html
This is better.
http://extreme.outervision.com/PSUEngine