If you are over 21 choose a hotel in the lower Sukhumvit Road area so you can use the Skytrain, walk to restaurants, nightlife, cheap hotels, etc. Under 21 stay in the Khao San Road area with other young travelers.
For hotels and specific information use the below travel forums:
Virtual Tourist
http://forum.virtualtourist.com/forum-1943-1-Travel-Thailand-1-forum.html
Trip Advisor
http://www.tripadvisor.com/ShowForum-g293915-i3686-Thailand.html
Lonely Planet Thorn Tree Forum
http://www.lonelyplanet.com/thorntree/forum.jspa?forumID=51
No rail service in Khao San Road area.
http://www.urbanrail.net/as/bang/bangkok.htm
Good luck.