The 430EX has a Guide Number of 43, the 580ex is GN=58, and the 600 GN=60.
The Guide Number gives you appropriate distance × f-number for each flash. So, the 430EX, with a GN=43, will illuminate a subject that is 43m/at 100ISO. If your aperture is f/8, and subject is 5 meters away, that is 5x8 or 40m, within the power range of a 430EX. If your subject is 20m away, at f/8, then you need 20x8 or 160 GN, well above even the 580.
If your subject is 10m away, and your aperture is f/4, then you need GN=10x4, which is well within the range of the 430EX, and likely typical of the shooting conditions most find themselves in.
Now, the trouble comes when you use modifiers, or are trying to illuminate a large scene, where you need more power to overcome the impact of the modifier or situation. This is where the more powerful flashes really help.
In my experience, umbrellas tend to preserve as much light as is possible, especially if you have a silver lined one. Softboxes are shoot-thru, and tend to reduce the light significantly, often forcing the softbox to be very close to your subject. If you had to choose, reflecting umbrellas will provide much more light with flashes than will softboxes.
But, that being said, I have 2x 430EX, and find them more than powerful enough. Two personal examples:
Scene 1: outdoors, dusk. Two 430EX, triggered by ST-E2, both using reflectors. ISO400, f/2.8.
Distance to subject was about 2 m.
Scene 2: Indoors, dimly lit church (as in dark), had to light a crowd of 40 people. Used two 430EX, 3 meters away, umbrellas, f4, ISO 200. Plenty of flash, in fact, the lighting is harsh due to the contrast. (yeah, lighting is harsh, but I had limited time, and conditions. Even had to shoot at 17mm)
So the 430EX have plenty of power. In my opinion, the only reason to get a 580 or 600 is if you want the off camera control.