Hi Jess,
I am about to embark on my 10th and 11th Thailand trip this year and I am still surfing the net and forums, overloading myself with information. We are more careful with planning these days as we travel with our two children and like to have our accomadation pre-booked so we dont need to trudge around looking for something while I am there.
If I was in your situation, however, I would book a few days in Bangkok at a nice central location to start with and then just see where the mood takes me.
There is plenty to do and see in Bangkok, but it took me until my 5th trip to truely appreciate it as it was so overwhelming.My favorite Bangkok market is Suan Lum which is near Lumphini park and station, but it is supposed to be closing soon and I cant find any solid information on when the exact closure is set for. I would definately recommend shopping and eating at the MBK as prices there are similar to most markets but pricing is a bit more structured and you are less likely to be ripped off as a firsr timer. Hire a long tail boat and explore the river and side canals of Bangkok to get a true reflection of life in the city. Go to Lumphihi Park and watch the locals on their early morning or evening exercise rituals, I know this sounds a little lame but I ensure you that it is truely an enriching experience. Eat and drink at Nana Plaza or one of the night markets in an outdoor foodcourt and beer garden and talk to the young locals and other tourists about their favorite places in Thailand before making your mind up where next to go.
Which Island location you go to should firstly be determined by the time of year. If you are going between NOV-MAY, then the Andaman sea side(Phuket/Phi Phi/ Krabi) has more stable weather but accomadation prices hike considerably. If the success of your holiday is based on hours of sunshine and spending time in and on the water, then it is worth paying the extra as there will be far less chance of rain and the seas are flat . The Gulf side (sumui etc) should be avoided from OCT-JAN for the same reason.
For our November trip we are gong to AO Nang and PHi PHI for our first extended stays at either location. Our budget extends a little more than yours but there will still be accomadation in your price range that will be clean and comfortable at both of these locations. At Phi Phi, have a look at a guest house called Cheap Charlies if you want a central location . Phi Phi is an absolute gem. I have only been there twice for 2 night stays but I have been sad to leave both times. You can still get a long tail boat to a quiet beach during the day and then immerse yourself in the Beach Party scene at night. Most negative comments of Phi Phi come from people that take a day trip there from Phuket and just see the crowded main beach/pier/village area in the 3 hours that all the other day trippers converge. It is a very different place most of the time and is one of the most beautful locations on earth.
Samui has always been my favorite but on my last trip there 2 years ago, Chaweng was a little bit busier than my ideal and proberbly yours from what you say. It is still a beautiful beach at the right time of year and if you felt it was too commercial you could skip over to Ko Phan gan which is starting to develop into what Samui was 10 years ago.
There are heaps of less developed beaches and Islands in Thailand , but if you are after a bit of night life,these are the ones I would go to for a great allround experience. I also love Phuket, but we prefer the less crowded southern beaches to Patong these days but there will be plenty of people on this forum that still sing the praises of Patong. The bottom line is that I cannot rememer having a bad day in Thailand no matter where I was or what the weather was. Go....Explore....Enjoy.

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