A good 700 watt should suffice but i would recommend no lower than an 850w..
The reasons for this means if you had hard disks etc youll have loads of room to cope..
a 1000w is designed for tri or quad sli systems......
the thing I would wait untill nvidias new card comes out in a month or so..
one new card will be faster than the old 2 and it will be dx 11..
SLi is a buzz word and not worth the money imho..