Hi Joel
I completely agree with VegasMike about Pattaya, there are many nicer places in Thailand. But, there are some very nice islands in that area, Koh Chang, Koh Samet, Koh Kood - might be worth taking a week to explore. I would then suggest taking the high speed ferry down from Chumphon - Koh Tao - Koh Samui, I lived on Koh Samui for nearly a year, and got bored because of the amount of foreigners there, but its got beautiful beaches, and the islands of the Angthong National Park are fantastic!!.
Phuket - Krabi - Koh Lanta - Phang Nga Bay - Phi Phi Islands - Similan Islands - all accessible from Phuket, again the same as Koh Samui I found Phuket just to crowded with foreigners around Patong, I stayed up in the north of Phuket, fab beaches much more Thai.
The Andaman Sea locations are without doubt far more stunning & beautiful than the Gulf of Thailand islands & beaches, but whichever you choose you will have a great time!!.
When you are ready to start planning the Bangkok - Chiang Mai section, catch up with me again to help you with that.
Kind Regards
Kevin