Your system isn't going to draw 550W, ever. If you buy a good (!) 450W+ PSU, you'll be fine. Don't be fooled by the numbers; a cheap 1000W PSU is more likely to kill your computer than an expensive 500W PSU.
FYI, I have a computer with an older AMD Athlon64 (3700+, single-core), a nvidia 6600GT videocard and three harddisks. I have a 450W Coolermaster PSU with a little power meter thingy. It says I'm using around 100W of power, idle and while gaming. (Note: I don't know how accurate that meter is!) The air coming from my PSU is barely warm, which is a good sign too.
I remember reading that you should get a PSU with a maximum output that is roughly twice the maximum amount needed by your PC. So, if your components need 200W, your PSU should deliver 400W. That'd be the most efficient setup. But please correct me if I'm wrong.
