depending on how serious of a lighting setup you're looking for, i would looks at it two ways. one, you go with speedlights. two, you go with monolights.
if you want a dependable light to use on occasion that will get the job done, speedlights are a good choice. they're fairly easy to learn and you will wind up finding a ton of instances where a single undiffused light will give you some really nice results. they're super portable and you can find AA batteries anywhere.
monolights are a good choice if you plan on using artificial light on a more regular basis. monolights also open up a new world of optional light diffusers. soft boxes, beauty dishes, snoots, reflectors, grids and so on. and while there are definitely ways to also use all these diffusers with speedlights, if you plan on using anything beyond an umbrella, i would suggest going with monolights. they're generally going to be much sturdier, making them able to hold heavier light modifiers.
i have two paul c buff 640w einsteins. they're very easy to use, are completely digital, and cost a little more than a canon 580ex. they also make very inexpensive lights that produce the same light that really expensive ones do. most people aren't going to need 640w's.