you don't need a visa for Thailand, you get a 30 day one on arrival. To get from BKK to Phangnan you fly Bangkok Air to Kho Samui and then a boat over to Phangnan, I think the return flight is about $350, but if you have only 3 weeks in Thailand it is not worth fucking around with buses and trains. Cheaper then BKK Air is to fly Air Asia to Surat Tani and then take a direct boat to Phangnan, a bit longer but the return flight will only cost you about $100, you can book either Airline on the net. Most beaches in Phangnan are beautiful and it is only a small island and you can hire a scooter for about 150 to 200 Bath a day

the airfare from bkk to samui is less than that, at least when i checked the other day... more like 220 bucks round trip i think. you can book it online (and should, the flights do sell out). www.bankgkokair.com -- i think another airline has gotten permission to fly into the samui airport lately, you should check into that.

once you're at the airport, there are three ferry options to kho phangan:

if you want to go to haad rin, you take the ferry from big buddha, which is close to the airport. it's a rickety big old boat, and takes about 40 minutes. if you want to go to anywhere else on the island, you take the lomprayah catamaran, which is faster and has fancy seats and aircon, but more crowded. there's a thread on www.thaivisa.com in the samiu/phangan forum that has the updated schedules so you can see when you want to arrive to make your ferry connection, and you can get your bus transfer and ticket right at the airport when you get off the plane, right where you pick up your bags.

i llike the east side of kho phangan for really chill beaches of tong nai pan and than sadet...that's over the mountain from the thong sala ferry landing, and the taxi is expensive... a year or two ago i stayed at mai pen rai bungalows in than sadet (super-chill, loved it, though no power at night so no fan), and they suggested it take this funky wonderful old ferry that goes to haad rin and then the smaller beaches on the east side, from a different ferry spot on samui... i loved it because it just goes as close in as it can get, and you have to climb out and walk through the water. but coming back, the weather kept it from running, which doesn't happen with the bigger boats.

a lot of folks like the north end, choluklum is pretty, and bottle beach is easiest to get to by boat so it's quiet. think about whether you'd rather see sunrises or sunsets, too, and how important swimming is... there are reefs on the west/sunset side that make swimming impossible. the east side swimming is better, but you can't see the sunset.

the northwest got built up fast, though some people love haad yao on that side, since there's a little more bar nightlife. again, reefs affect the swimming. and kho ma is a little tiny island off the northwest corner that has a few little places near it.

not sure how long you're staying, but you might want to book a couple of nights somewhere central and quiet like baan tai (i stayed at the dewshore bungalows a year ago, they had a couple of old funky on-the-beach bungalows, and a huge new pool... though i think they were planning to replace the old bungalows with concrete ones), and then do one of the many boat tours around the island (you'll see lots of posters about those, and your bungalow can help you book it), and then you'll see all the good spots, and can choose a place to move to later. or not. kpg is like that sometimes. it's the hammock life ;^)

if you're a party person, the sunset side of haad rin isn't bad, and it's easy to get to the sunrise party beach, but not in the week before and during the full moon, at least not for me.

www.thaivisa.com has a lot of up-to-date thailand info.

and no, you'll get an automatic 30-day visa exemption stamp on entry, you dont need to get one in advance.

feel free to pm me if you have any other questions. i lived on the island for a year five years ago and have returned many times since.