Hi ... just found this thread ... it has been put in the chat forum ... easy to get to either thru home page or down bottom on the 'jump to' bit.
You will get loads of info from this forum ... very helpful and friendly forumites! I only joined in January I think and got all the info I needed in only a few short months (if anything - I got too much info and didnt have enough time to do it all - only 2 weeks)
Is your $1500AUD saved also for accommodation, or is that just spending??? For a 3 week trip, that wouldnt even be enough just for my spending money!!! Accomm you will find very cheap tho.
Have fun planning and keep asking q's! Read all my jbr's from recent trip. Also checkout this website from a forumite - its got loads on info on lots of asian countries - also Cambodia
www.cathyandgarystravelpages.com 
Kitty
PS Ive just realised your thread has actually been put into the "SEA travel help and information bulletin" and not the chat room.