The specs has only changed for Horizons, so you should be able to play the original part (Season 1) like before. Some time later next year you will have to use 64-bit for everything but as far as I understand it you are not forced to upgrade your graphics card if you stay with season one. I also understand, that both clients stay separate but use the same servers. Otherwise it would not be possible for Horizon-Avoiders to still play the original game. So you should be fine to play the original part as long as you want but will to have follow the new specs as soon as you want to use the horizons exclusive content.
ok thanks for the update on that madman as i said hope your right and no offence but would be nice if someone from FD could confirm this just for peace of mind if possible.