Pattern dictated the chip after Pentium should be Hexium because they're using the greek prefixes rather than the latin ones.
Of course the media and the general public find sexual innuendo to be much more amusing, (and not like anyone knows any better...) so they speculated Sexium, and presumably Sextium instead.
Sex(t)ium would come after Quintium. (or quinium) Who'd name anything quintium??