I'm back after some hard working on my netbook. I will not speak about things which worked just fine, but only about my issues.
The first one is the most important: I couldn't manage to have a working 64bit system. So I got back to a 32bits system with:
CFLAGS="-02 -march=native -pipe -mmmx -msse -msse2 -mssse3" (some of these options are only available after an update to sys-devel/gcc-4.3.2-r3)
Then during the kernel configuration, don't forget like me to select the PATA drivers for the Via chipset. It took half an hour to compile with the NC20.
After that I was able to boot under my new Gentoo. The next step is to recompile system tools. An "emerge -e world" will do it.
I will be back soon...