What version of release of contributed modules to use with certain Core Drupal core release?

scenario
one decided to use Drupal 5.0-beta2

so release versions he/she needs to use for instance of Content Construction Kit (CCK)

http://drupal.org/node/48429/release
cck 5.x-1.0-beta
cck 5.x-1.x-dev
cck HEAD

or if he/she decides to use
cck 4.7.x-1.0
cck 4.7.x-1.x-dev

default release is
http://drupal.org/project/cck
4.7.x-1.x-dev, 29/11/2006

or indeed id he/she wants to opt for version 5 of the core

what is the more sensible to download
Drupal 5.x-dev?
or
Drupal 5.0-beta2
I know its been mentioned many times within lengthly posts but there is not 1 clear paragraph to sum up the differences risks and advantages and what versions work and what are the differences for firstcomers - as much as i know

dev. means is development version under development another means not stable' right? or wrong? if so why most of the default modules doenloads are
4.7.x-1.x-dev
http://drupal.org/project/Modules/date

what is the difference between Dev. and Beta exactly?
if Dev. is "Nightly development snapshot from HEAD"
so why dev versions are default download?