Just a word of caution about travelling by bus.... ESPECIALLY AT NIGHT and ESPECIALLY buses departing from Khao San Road area.
On the old travel forum (the one that existed before this newer site) there was quite a lot of discussion about this..... apparently thieves operate on these buses and they were/are very well organised and very competent at what they do.
While you're asleep the gang members on the bus steal your belongings from your bags while other gang members are in the baggage area (the stow area) picking locks on cases and bags.
Travellers arrive at their destination early next morning and when they go to check into their accommodation, find they have no money and no passport.
Day bus journeys may be better.