Hi G-Stirl,
Welcome to the Forum.
SS 2008 is still doing well even though it has been superseded by SS Extremes. There are almost 500 missions available for SS 2008 but you would also need to have the add-on called 'New Horizons' to play all the missions.
SS 2006 has lost its sheen a bit and we rarely see members asking for support for it. In my opinion it would be advisable to upgrade your current PC so that you can enjoy the full potential of SS 2008.
Regards
vin_sun