You will probably get this information too late to help you but perhaps someone else can benefit from it.
For a visa or if you are visa exempt depends on the passport you travel under. If you are traveling under a US, UK, Canadian, Oz, NZ passport and many places in Europe you will be able to get a free 30 day entry stamp in your passport at the airport on arrival. If you arrive overland you will only get 15 days. For a short while Thailand is offering a free 60 day visa that you have to apply for ahead of time. http://www.mfa.go.th/web/2482.php?id=2490http://www.mfa.go.th/web/2482.php?id=2491http://www.thaiembassyuk.org.uk/newversion.../visa_main.html
If you have a hotel near the Skytrain route you can use that to get around the city. The Skytrain will also connect you with the subway at Asok. The subway goes directly to the national train station.http://bangkok.sawadee.com/skytrain.htm
From the airport you have a choice of buses and even meter taxis. When you use a meter taxi in Bangkok the price you pay is for everyone in the taxi not per person. After midnight no Airport Express Bus until morning. http://www.bangkokairportonline.com/node/55http://www.bangkokairportonline.com/node/52
When taking a meter taxi in the city you do not have to pay any surcharge going back to the airport. Tell the driver to use the meter. If the driver wants to bargain the fare you will pay more. Most will gladly use the meter because there are hundreds of other meter taxis that will use the meter! If you wave a moving taxi over they are less likely to want to bargain the fare. Too often taxis parked outside hotels and bus stations want new visitors to bargain the fare and you pay more! Have small bills for paying for taxi, they may claim to not have any change hoping you will be stupid enough to say keep the change. (Meter starts at 35 baht and a meter taxi for tourists is cheaper than tuk-tuks too!)
Wait until you get to Thailand to change any money over Thai baht. Bank money changers are open 24 hours a day at the airport both coming and going. Also there are ATMs at the airport. Travelers Checks are easily cashed, you have to produce your passport. Travelers checks get a hair more than ordinary cash - there is a 33 baht fee to cash a TC so don't waste lots of fees on small amounts. There is a 150 baht fee to use an ATM. For the same amount of money a travelers check can be cheaper.