I would suggest you download the latest 4.2g (gnome) or k (kde)
Your pentium... I would assume is a 32 bit cpu
The amd64, would be for a 64 bit cpu
You also have the option for the core cd which will give you no x, and min. network after install. Is for more advanced users.
Since sabayon is a rolling distro a lot of releases are being added to the mirrors, as sabayon keeps moving forward.
Is a daunting task to keep all the mirrors up to date and remove all the old releases, I personally like to have a few old releases around for certain problematic machines.