I just had a 6 hour layover there on the way home from Bangladesh a month ago. Depending on the time (of day) of your layover, going into town is probably not the wisest. There is not a train, so you must take a taxi/bus/limo.
I suggest going to the Day Rooms. For about $67 USD, you can rent a room for four hours with a shower, bed,
TV (channels only in Thai or displaying the airport arrivals/departures), toilet, and mini-bar. It also includes breakfast (although I did not take part in this, as I was not feeling well). The only problem with this is that the internet is not free in the Day Room. There are also rates for 6 hours, 12 hours, and perhaps 2-3 hours.
The Day Rooms are located at the end of terminal G. It is on the same level as the duty-free shopping. There is an abundance of duty-free shopping, so you could knock your socks off there and buy new ones. There are also a few restaurants.
[Edited 2007-12-10 20:08:23]
Private Pilot, Airplane Single Engine Land / former frequent flyer with 9 straight years being elite on NW/DL