FZ8k - the "k" stands for black, so there must be a "s" silver model too.

As one reviewer put it.
The FZ18 is not a replacement for the FZ8, it is more of a big brother.

Here is a link to a review of the FZ18:


It spells out all of the differences between the FZ8 and FZ18 on the first page of the review.
Only you can determine if it is worth the extra cost to you.