550 will be no where near enough. A 5970 draws slightly more than 2 5850s which should have a 750W PSU to support them. Remember that even though the 5970 is a single card it has the power draw of 2 because its dual GPU. A good 750W psu should be enough for it, but only 1 of them, if you are thinking of possibly adding a 5870 down the line from trifire you should consider a 900W PSU.
Thanks for the quick response. I don't ever see needing to Trifire (especially with the CPU bottleneck), but in the interest of giving myself a little overhead this Corsair TX850 seems like a good choice (and it has a nice MIR right now):