It depends on how many, and how powerful, the hardware in your computer is. if you only have 1 hard drive and a CD drive, you might only need 300watts. If you're a real geek like me, you have many hard drives, multiple CD/DVD drives, a high powered video card, etc. In that case, you will need mroe power.
First, you need a power supply with all of the connectors for your hardware. For example, not all power supplies have connectors for PCI-Express video cards. Most of them do, since PCI-E is so common. Something else to watch out for is the type of connector on your motherboard. Some have a 20-pin connector, some have a 24-pin connect. When I buy power supplies, I try to buy one with a 20/4 pin connector. This gives you the option to detach the 4 extra pins so that you can plug it into a 20-pin mobo. Flexibility is nice :)
Also, if you plan on running a multi-video card setup like SLI or Crossfire, you need a power supply with multiple PCI-E power connectors coming from it. So once again, make sure the power supply has enough connctors for all of your components. There are adapters that allow you to turn 1 connector into 2. The danger with this however is that the multiple pieces of hardware connected to the 1 power line may try to suck up too much juice if they are all running at once. In my computer, I split 1 power rail into 3 to run my CD drive, and 2 of my fans. The fans use hardly any voltage, and the CD drive isn't always spinning. I would NOT recommend splitting 1 power rail for 2 hard drives though. If they both spin up at once and suck up too much power, you could end up crashing your computer a lot.
Aside from having enough connectors, there are many other features built into the power supply that help gaurantee a good, stable supply of power. There are also feature to help protect your hardware from "accidents." For example, it is possible for a power supply to go "bad." When this happens, it could maybe send a surge through your equipment, destroying a lot of hardware. There are Power Supply Units that protects against this sort of thing. Also, some of the cheap-o ones don't deliver stable power. Monitoring voltage, I have seen the voltage on some rails go up and down like a yo-yo. For some people, this is no big deal as long as there's enough power. However, I dont' like spending $1000 on hardware and then giving it "dirty" power. If you think of it like a car, it's kind of like putting dirty or watered-down gas into your tank. You wouldn't put 83 octane into your new Porsche would you? And even when you filled up with the good 92 octane, you probably wouldn't just run down to Biff's Corner Gas for it either. You'd probably go somewhere you really trusted :)
The power supplies you mentioned are VERY nice, and I would trust them in any system that I built. However, for most applications they are overkill. They are very expensive because of all the little features, such as anti-vibration and "turbo mode." You have to ask yourself "are these features that I really need or want? Will I use them?"
I'd say the most important things to look for are A) enough power connectors for your equipment, B) enough power, that is stable and, C) a good company with a good reputation. Personally, I buy Ultra power supplies when I can. From my experiences in tech Support, they have excellent customer service, and most of their products have a lifetime warranty. Steer clear of the crappy companies, like Enermax, Apevia, and Diablo. Look for good companies like Tagan, Ultra, Thermaltake and Zalman.
Below is an excellent article feature 15 power supplies. I love Tom's Hardware because of their many benchmarks, comparisons, and good accurate reporting. They are very hands-on and very reputable.