How much of a power supply do I need?

I have an Antec 550 watt power supply. I have the following setup: amd64 4200 dual core EVGA 256-P2-N517-AX 7800 gt 5 hard drives (3 sata, 2 ide) 2 dvd burners (1 sata, 1 ide) and 8 case fans. 4 are standard (.10 amp each), 2 are blue led-lit thermaltake case fans (.48 amp each), and a vantec tornado (.9 amp,... show more I have an Antec 550 watt power supply. I have the following setup:
amd64 4200 dual core
EVGA 256-P2-N517-AX 7800 gt
5 hard drives (3 sata, 2 ide)
2 dvd burners (1 sata, 1 ide)
and 8 case fans. 4 are standard (.10 amp each), 2 are blue led-lit thermaltake case fans (.48 amp each), and a vantec tornado (.9 amp, but is kept at low with a variable resistor dial), the other is the standard HSF on the processor.
It seems to run fine this way, however I want to get another of the same video card so I can run SLI.
Do I need a kW or what?
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