the second u add that second GPU your CPU will be bottlenecking your system.
given the fact your CPU wouldnt be able to handle two gtx 480s, i dont even want to imagine what would two gtx 580s would do to it.
no matter how high u will get your CPU it still gonna be hardcore-bottlenecking your system.
my advice, if u wanna go for 2way gtx580 SLI, you need a MUCH more powerful CPU, a 6.4GT/s CPU to be more precise.
whatever u decide,
GL and HF doing it
PS: dont even think about going AMD with gtx580 SLI!