First Portrait Shoot

I shoot a lot of commercial still life, sports, outdoor wildlife and landscapes.
I have just been asked by a bunch of parents from one of the teams I shoot to shoot family portraits for them. Hooray $$$

What sort help can anyone give me on shooting these families of 3 to 6 people in each family? The shoot will be in my house with my 20D. There is no lighting coming from the outside.

I have been experimenting with different speeds, fstops, flash settings, distance settings etc. and have come close to getting good color and light but not perfect (the wife has been very patient).

I have some of and plan on getting the following items.

580EX bouncing off wall behind me mounted to 20D.
75w Spot pointing down from 7feet slightly behind and off to the side to make hair shinny.
Umbrella with natural light 75w bulb and flash activated promaster flash
Backdrops.

I need help on shooting modes, distance and maybe the best lens (fixed) or will my zoom 17-85 be good.

If anyone needs any further info to help provide answers, just ask away.

If you're going to be doing portraits you'd probably be much better off with studio lighting, since it's a lot more powerful and flexible. The cost is about the same as a 580EX, you can find recommendations here. A single 580EX bounced off a wall behind you will probably make very flat photos, though with groups the lighting needs to be pretty uniform anyway, to avoid dark/light spots.

As for lens, you should use your 17-85 to work out what focal length you'll need, then choose a lens. I don't know how good the 17-85 is, but most lenses are find when stopped down to F8 or so. The only issue is lens flare and not sure if that's ok with that lens or not. The 18-55 kit lens is terrible for flare.

Constant lighting isn't very bright compared with flash, but as a hair light it might be ok, you'd have to try it out to be sure.