I have a 650W and it's fine, though it's a high quality one (Antec EarthWatts 650W). Note that some cheaper/low quality power supplies may be rated by maximum peak power output and not continuous, this means that the rated number can be provided only for a short period (for instance, at start-up where all the devices suck down huge power to start up, especially motors on fans and hard disks). A good power supply is measured by continuous power output, which means that the rated number is the amount of power that can be provided for an unlimited time. The HD5870 has a continuous maximum power draw of 175W while fully loaded. Take your CPU as 150W (typically less, but it's a rough estimate), your motherboard probably draws another 50-100W depending on chipset and RAM, hard drive probably draws 50W or so, fans maybe 10. Add them up (175+150+100+50+10 = 485W) and you have some extra room for power spikes, CD drives (really only draw significant power when spinning/reading/writing a CD), and other accessories such as USB devices which draw power from the PC. I think your 550W should work for a single 5870 build, a 650 would probably run a build with 2 5870's in CrossFire, and a 700+ would allow for 2 5870's with some headroom for more hard drives or other upgrades.
No nVidia card supports CrossFire. CrossFire and CrossFireX are ATi technologies. SLi is nVidia's competing technology. While they serve the same purpose, they are different and must be treated as such. A CrossFire motherboard will not support SLi and an SLi motherboard will not support CrossFire. To have a choice, you must get a board that supports both, but I don't know if these are available for the AMD platform (Intel boards using the X58 chipset support both, though only one may be used at a time).