Flash unit guide numbers.... confused!

[woody]

I'm probably being a bit thick with this question but...

Can someone explain how the guide numbers work? - I found this on the Jessops site:

jessops web site said:

Guide Numbers are used to calculate the correct exposure of a flash unit. The guide number, divided by the subject to flash distance (in metres), equals the aperture required for the correct exposure (but most flashes are automatic).

I've noticed that most are given at ISO 100 - how does this work if you have your camera set to a higher ISO?

The reason I'm asking is I want to get a flash unit for my 300D - but don't want to get the wrong one for what I want to do. I want one that will be OK for taking to Le Mans and using at night by the track.

Any suggestions as to which would be best?

Cheers

Chris