This sounds like a pretty nice travel plan to me Den. 3 nights in BKK will give you long enough to see the major sights. I always suggest a boat trip on the Chaophyra River, either take an organized dinner-tour or DIY on the local "river buses" Most of the important BKK temples can be seen/reached from the riverside. A week on Samui/Phangan will be fun, you can soak up the sun during the day and boogie at night (I'm much older than you and I still like to boogie) I'm not sure where Salad Hut is, Koh Phangan does have a bit of a rep' for rages, so there could be some action there. My advice is to join in. There's no need to spend big on food, plenty of cheap eating everywhere in Thailand, just ask to look at the menus first. I always ask how much a beer is, that seems to be a good yardstick for the rest of the prices. If a large Henieken is 75-80Bt, OK. If they tell me 150bt, get stuffed, I walk on.
I also suggest avoid doing heaps of shopping during the 3 days in BKK, I'd be inclined to work out what I wanted to buy and grab everything on that final day before flying out. You don't want to be carting heaps of stuff on the ferries.
Hope you have a ball.