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As long as you have a decently sized PC Power and Cooling I think your current power supply could handle the load of an SLI system. You would need two PCI Express power connections unless they make some sort of adapters to use some existing connections, which I think they might. I don't think the connectors are really that complicated so you could probably buy the connectors themselves and splice them into your existing power supply just fine. PC Power and Cooling could probably add them to your existing power supply for a fee as well.
most of the pcie cards w/ a needed power connection come with an adaptor for the new connection (the adaptors use 2 molex connections). so if you think your current psu would be ok you can just use 2 adaptors for the actual cards.