The 5MP images from the 5700 are excellent, and the camera is currently
good value for money.

However it does lack the image stabilisation of the FZ20 which is most
effective and gives you about three stops in shutter speed.  Conventional
wisdom for hand-held operation is that for a 280mm lens (like the maximum
zoom of the 5700), you need a shutter speed of at least 1/280s.  Image
stabilisation allows you to hand-hold the camera at about 1/17s instead,
and this allows photography in much lower light levels.

Add to that the f/2.8 aperture of the FZ20 (compared to the f/4.2 of the
Nikon 5700) which allows even lower light-level operation and better bokeh
(out of focus) effects.

I have a Nikon 5700, but we bought a Panasonic FZ20 as an upgrade for my
wife's Nikon 990.  Very pleased with it.

(Other cameras you might want to consider are the Canon S1 IS if you are
happy with 3.2Mp, and the Nikon 8800 if you really /must/ have 8Mp,
although the 8800 has a small maximum aperture).

David