Found this qoute from a dell website by a customer, just seeing wwhat you thought
On the bright side Dell is known for underrating their powersupplies. I only have the regular 6800, but I don't imagine that its too much different as far as power consumption. However, my computer has a lot of cards/drives/extra fans, etc and I have never had a problem with power. I suggest testing it with your current power supply and seeing how it goes.
so, do you think i can buy the 6800gs now, then overhual everything when the g8 series comes out?
or will the 6600gt be a good bang for my buck. like as in, how much performance increase will i see over my ati x600 256 mb card. can i play most new games on atleast mid settings with some AA or soft shadows.