If you have a current FAA licence with an A320 endorsement you will have no problem getting a CAAS licence, as long as you are to be employed by a company on the Singaporean AOC eg Tiger, who must do your training. You will need to do the Air Law exam, and the A320 technical. Both are piss easy if you have the cheat sheets that are floating about.
Make sure you have, before you arrive:
Licence
Stamped logbook
English Proficiency test
Current ME CIR
Proof of HPL exam
Supporting paperwork from the Airline you will be working for ie Tiger.
All current, all on the same licence. They will not accept bits and pieces of different licences.
Each individual case seems to be treated slightly different, it's just a matter of digging up all the crap they want but if you have all of the above you will be ok.