It really depends on how and what you shoot. If you have a small space, the lower power lights will be fine.
If you need to fill a large light bank with light, you will need more power. My power supplies range from 200 to 2000 in the Norman line, 1200-2400 in the Balcar Line, and all 500's in the Ultra line. I use the ultras for accent lights mostly and main and fill for portraits and weddings on location. I have five of those. I hope to have about 10 when I am done so I can do more of the Playboy Centerfold style shoots. You can never have too many lights on locations, unless you are the one who has to carry them.
When I am shooting 4x5 or larger or table top images, I use the larger packs. Can't get f:45 out of a 500 unit in a soft box unless you set it on the subject.