<p>I'd like to install Gentoo. I need it to develop GUI C++ applications using wxWidgets, so I need:</p>

<ul>
<li>build tools: make, automake, autoconf, etc.</li>
<li>C++ compiler (GCC)</li>
<li>X Window System for testing (Fluxbox or something minimal would be enough)</li>
</ul>

<p>Now, I have two options:</p>

<ul>
<li>download the small network installer (57MB) do a network install</li>
<li>download the 600MB CD</li>
</ul>

<p>I'd like to download as less as possible and still have all the tools above.</p>

<p>I also don't understand whether the network installer will first prompt me for the packages I want, or it will fetch 600 MB of data anyway?</p>

<p>I might want to install it on other computers later, so I'd go with 'full' install from CD if the network install does not save me anything.</p>

Gentoo is ultraminimalist by default.
The install CD gets you a basic working system, a basic compile environment ( Some version of the GCC suite ), and package management.
Its up to you then to install what you want to use.
Its not like many other distributions where theres a big set of "default" packages to have installed.
You have to know what you want, and install what you want.
The "Live" Cd will make things a bit quicker  by having a few precompiled binaries available, but besides that, you still have to choose what you want to install.
I also don't understand whether the
  network installer will first prompt me
  for the packages I want, or it will
  fetch 600 MB of data anyway?
it will only install what you want to. If you use NetInstall and install nothing except GCC, it will only download enough to have GCC.
Welcome to gentoo.
It can be a little daunting for first timers, but once you've gotten past the steep learning curve you'll love it :)