Jack Neubart wrote an interesting Studio Flash Lighting Tips article at Shutterbug. He explained that it’s not necessary to buy everything at once. In fact, many pros succesfully work with only one or two lights. Moreover, he also wrote tips on matching your lights and lighting accessories to your needs:
“Studio portraiture. The lights don’t have to be more powerful than 400 ws. Often, one light will do, plus a white photographic umbrella (which produces a softer, more neutral light, compared with silver umbrellas, which tend to go cool). Add a second light for the background or as a hair, accent, or fill light. In a pinch, the camera’s built-in flash will do as fill, while also triggering the studio strobes (via their built-in slave cells). More powerful lights, say 800 ws, will ensure greater depth of field. You’ll only need something even more powerful, for even more depth of field and with faster recycling, if you’re shooting fashion or groups of people. Now, the question becomes: power pack system or monolight? The simplest answer is a monolight. One often works wonders. A flat-panel AC monolight is great for head-and-shoulder portraits, with a fill card to reduce contrast.
Tabletop/still life/food. Stopping down to small lens apertures is often required, though not always. You should consider one 800 ws light for starters, more if you can afford it. A softbox is a wise addition. As your photography grows more sophisticated, you’ll add to this, with more lights and such things as honeycomb grids and barn doors, both of which control the spread of light. You might also want a diffusion housing (a cube-shaped or rectangular light tent, for instance) to further reduce contrast. I’ve found flat-panel self-contained strobes work well here, and these lights don’t require a softbox or umbrella. “
Read the complete Studio Flash Lighting; There’s A System To Fit Every Need article at Shutterbug.