DV Info Net - Lens focal length question

Please excuse what might be a very naive question!

I am looking at the possibility of buying either a Panasonic AF100 or a Sony FS100 (or even, possibly, the FS700). This will be my first foray into the world of interchangeable lenses. (Actually, not strictly true as I used to own and love an XL2, but just with the stock lens).

My knowledge of lenses is somewhat limited and I know I will need to do some research, but in the meantime my question is this:

I own a Canon XHA1S which has a zoom lens with a focal length of 4.5-90mm. I understand the concept of dividing the higher number by the lower to get the equivalent x zoom, in this case 20x (and I also understand what a relatively useless number this is!). What spec zoom lens would I need to have roughly the same reach as the Canon's longest telephoto? Would it be 90mm? I suspect that there is a lot more to it than that. Can anyone give me some pointers please?

EDIT: OK, I've started my research and I now realise that the other key factor is the focal length relative to the size of the sensor, which is clearly going to be much larger on both the AF100 (Micro 4/3) and the FS100/700 (S35). I should probably ask this question in one of those forums, but to avoid the wrath of Obstreperous Rex by double posting, I will leave it here for a little longer before asking elsewhere! Answers to my original question still welcomed!