here's another:
http://patriot.net/~jonroq/Tech/octane2.html
FYI - if your car doesn't have electronic mgmt, you need only an octane level that prevents pre-ignition. More octane will only coast you money. If it DOES have electronic management with knock sensors, then your engine MAY benefit frm raising the octane level. My Porsche Turbo couldn't care less, but M5 loves the stuff and makes more power with it.
Do NOT use the car parts store "octane boosters" - they are a sham, and (according to Steve Dinan, one of the most respected BMW tuners) they contain some harmful additives. Increase octane by buying it at the pump if you can, or with toluene (see the above link.)