For a single core and SLI 8800GT's with everything overclocked 40A on the 12V is more than enough.
For a quad-core 40A is probably still enough but depending on how many optical drives and HDD's you have 43-45A would be a better bet to give you upgrade and headroom.
the 550VX from Corsair will be plenty enough and is a very good choice for your system.