The manual for the FZ20 has a handy-dandy chart that lists the number of pictures at different resolutions and qualities that can be stored on different sizes of memory cards. You can download it from panasonic.com.
Problem w/ your question is that you're probably assuming every camera is the same for file size. That's just not true although they might be the same for a given MP count at a given quality setting for most cameras for a given brand -- or the ratio might hold reasonably well for a given brand.
For example, Nikon's max quality (Fine) JPEGs typically yields smaller file sizes than Canon's max quality (SuperFine) JPEGs. Now, when you're asking about Fine 3MP JPEGs for the Canon SD110, are you really asking Fine or SuperFine?
Anyway, why not do a little homework and go check it out at one of the popular review sites? Either DPReview.com or Imaging-Resource.com will likely have your answer.
And oh, one more thing. File sizes can vary a lot depending on the exact contents of the pics. You are more likely to get large file sizes w/ highly detailed pics like for landscapes w/ good daylight than for people portraits and/or indoor pics (unless you boost the ISO and introduce lots of noise/grain).
Personally, I'd rather purchase multiple 512s than go with the gigs. We lost some honeymoon pics on a card and thank goodness we didn't have everything on one card.
The bad card was a Kingston and we were unable to recover 200+pics from Frankfurt and Barcelona!
I use nothing but Lexar cards, and have had no problems with them in any way. It has a very good image recovery program built into the reader that they all come with.
I bought a Lexar 512 CF (12x) for my camera, and within a few eeks it is giving me erros and not writing files to disc after a certain point. The RMA process took a week to get a return authorisation. With fall here and a turnaround of a t least a couple of weeks, I have been limping along, but my first experience with this type of media hasn't been very reassuring. I would have to say that if given a choice, I will be buying more, smaller cards, so that if they fail, I at least have backups, and should I lose files, they will be smaller in number.