If you're into the nightlife I suggest you checkout:
Kao San Road, cheap beers, cheap hotels, but tons of farang backpackers
Nana Plaza, some hot girls, good for the naughty nights out
Silom Patpong, this is a mixture, some girly action but also some good bars and clubs, along with night market and good restaurants too
Soi Cowboy, some really hot girls but be careful some of the girly bars there are known to "pad the bill" with extra charges
Saphan Kwai, really cheap girly nightlife, some decent girls
RCA, pretty nice area with good clubs, some outside sitting along with some inside loud dance clubs, some super hot hi-so thai girls
Sukhumvit 11, lots of late night clubs, also famous Q Bar, Climaxxx (late club), and the old Bed Supper Club was there (it's closed now though)
Ekkemai/Thonglor, really nice bars like Muse and Funky Villa, i love this area for starting off the night
Good luck, enjoy BKK when ur here man