+1 on previous answers. Monolights require a power source, so if you expect to be away from such a source, they may not be for you. If you are looking for a kind of hybrid solution, Paul C. Buff (Alien Bees guy) has a portable power supply, the Vagabond (http://www.paulcbuff.com/vagabond.php) that gives you AC for a monolight in the field. Be aware that you are 1) adding gear, and 2) the number of flashes on this power supply will be limited -- by how much, I don't know.
If you mix a "smart" speedlight like the 480 with a moonlight, just because you'll have to run the monolight on full manual. Otherwise, the 480 is free to change exposure and you won't know until you've made the image how that interaction worked out.
In large measure, light is light. If you don't have enough, then buy bigger (i.e., studio lights, as you're discussing). If you want more shape but feel your power is ok, then just add another speedlight and have done with it.
One final caveat: Modifiers will reduce the effective light output of your speedlight because you are either bouncing or diffusing it. Either way, some light is dissipated, so whatever strength of light you have without the umbrella, you will have less with one.
A number of highly respected photographers work primarily with speedlights to great effect. Joe McNally and Rick Friedman come to mind immediately.