softbox is a modifier that you put on the flash/strobe unit.
Question you probably should be asking is which strobe units you should be looking into? - that's the fun part b/c there are number of cheaper units out there: slow recycling time, lower power outputs at full power, etc and number of more expensive units out there...
Then you just get a bracket and a softbox and yank that on the flash unit. Now yes, there are different sizes of softboxes out there and with different modifiyers on it but again, that's the least of the issue - first you need lights.
Food for though: if you have 2-3 speedylites, you can use a bed-sheet; tighten and attach it to something, shoot through it and it'll soften the light. I actually got this idea from Joe McNally's Hot Shoe Diaries.
Good Luck
