Tips for Thailand needed!!

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Hi everyone,
Im planning on flying to Thailand on the 30th August, a few days before the full moon party, which is on the 4th September. Im not quite sure how long I need to spend in Thailand, I was planning on spending about 3 weeks there, but wonder if thats too long to spend there?.
The places I want to visit are: Koh Phangan for the full moon party, so probably for about 4 days, Koh Samui, Phuket, Phi Phi and Bangkok for a couple of days before I fly out.
Could anyone give me suggestions on how long I would need to stay in Thailand for to visit and enjoy these places, and also could you tell me how much money I would need to bring. I will be backpacking, so wont be staying in expensive accomadation, but I plan on going out partying as much as my body will allow :p and I do want to stay in relatively clean and decent accomadation.
Any suggestions would be greately appreciated,
Thanks,
Grace.

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