Considering you're going to be going through the hassle of setting up softboxes on stands, I would strongly recommend that you should go with a studio flash setup instead of a hotshoe flash. You'll get a hell of a lot more bang for your buck.
Hotshoe flashes are great for what they're designed to do, which is to be something that you can mount on top of the camera. But you pay a premium for that type of miniaturization, in terms of both performance and price.
A Paul C. Buff Alien Bees B800 costs about as much as that 430 EX II, and it puts out so much more light that it's not even funny. The Buff Einstein flash, their flagship model, is cheaper than the 580 EX II and is ludicrously far superior in every way except that you can't stick it on top of your camera.
Don't fret too much if you're going to be shooting at locations without power. There are battery packs for most studio flashes. Buff sells the Vagabond which is good for hundreds, if not thousands, of pops with the type of setup you're describing on a single charge.
I'll also note that softboxes only work their magic when they're so close to the subject that you're having trouble figuring out how to shoot around them. It's a geometry thing...a 2' softbox is going to have to be no more than 2' away from the subject to be truly effective, preferably less, and it's not going to light up more than the person's face. a 2' softbox at the photographer's position of a 10' working distance isn't going to be significantly different from on-camera flash. A 5' softbox 5' away from the subject is going to be as effective as a 2' softbox 2' away, but it'll light up the whole person...and you can put that 5' softbox 3' away and get some amazing soft and even light wrapping all the way 'round your subject.
Buff sells parabolic reflectors...basically high-tech umbrellas. And they go from 4' across to over 7' across, and they make diffusers for them that make the light very similar to a softbox....