The two cards are not needed, unless your trying to impress someone. First off, are you building this machine mostly for games? Anyways, yes 550watts should be enough for SLI. Nvidia recomends 400 watts Min., 22a, 12volts for one card. I think at least 600w power supply would be better under a heavy load.
Now, Those power supply specs are for the entire system, not just the video card. If you did not have that duel core I would say you would be fine as you have about 30A of +12 available (I did not look up you power supply but typcially due to the Total Power Output ratings you would not get all of the 18A + 18A = 36A of 12V power.

we can worst case it, the new PCI-E 2.0 spec allows 300Watt per device installed in a slot. Thats 300/12 = 25A.

However up to 75W is supplied by the power going to the slot itself, leaving 225W max through the PCI-e power connector from the power supply. The slot power comes from the motherboard power connector and/or the aux 12V so 225/12 = 18.75A absolute max current draw for a PCI-e connector going to a single video card. (This assumes the video card bothers to conform to the spec)

I seem to remember one of the 8800GT reviews saying the card draws about 130W which is about 10A.

So the bottem line is do you really need two cards? For now I'd just use one and see how things run. Save some
$$$$. Next year, you may want two cards with the newer games and the prices will drop on those cards.