As you are undecided on your graphics card its impossible to guess,
are you overclocking anything, or intend to?
it may be worth your while reading the Psu sticky and/or using an online Psu calculator once you decide what card you're gettinghttp://www.tomshardware.co.uk/forum/284314-10-power-supply-guides-infohttp://www.thermaltake.outervision.com/
Assuming you are going to be overclocking and have a powerhungry card, I'd say you are looking at a 650/750w decent brand unit at least
**yup, the 6990 links I'm getting all say a minimum of 750w**