A couple of interesting sources, also follow links in page:

http://www.irc.nl/page/29189#information

http://dfwnetmall.com/energy/rainwater-catchment/first-flush-diverter.htm

Suggests a first-flush setup may do little good:
http://www.harvesth2o.com/first_flush.shtml

A complicated version:
http://www.slowsandfilter.org/first_flush_diverter.html

http://www.harvesth2o.com/RainwaterHarvestingManual_3rdedition.pdf

A thesis exploring various forms of first-flush. evidently the agree upon terminology, devices. An interesting read but a bit tedious:
http://rainwaterharvesting.tamu.edu/Justin%20Mechell%20Thesis.pdf

Reading/scanning most of that I'm struck by how little hard information there is that is applicable to any system any average homeowner is likely to build. Also how much overlap there is in sources, and how much deciding on a system is going to be pretty much like it was twenty years ago when it was handymen and backyard tinkers assembling systems based on their own intuition and estimations.

Yes there are some guidelines and lots of good ideas and things to think about.

I suspect that the safest course would be to use collected water for drinking only after it is chemically treated. I might give it some basic testing. Or simply make coffee or tea out of it so there would be far less chance of microbes being an issue.