Good response ste v :)
Bangkok is a really vibrant city which people seem to either love or hate. I lived there for a couple of years and personnally I loved it! I'm wasn't into the sleezy red night scene, but hey, if it cooks your goose...
If that is what you're after, then Patpong is the main area. If it's still there, there is a brilliant nightclub called Lucifers half way down the main strip. I'm going back a while but it used to play great tunes and had a really good vibe, with none of the bar girl hassles of a lot of the other places. (If it has changed since I last went, as things do quite often in Asia, then I appologise!!)
Otherwise there is loads of good stuff to do in Bangkok, ranging from ubber trendy drinking holes, copy english/irish boozers to cheesy karaoke bars. Bangkok is also a very reasonable city so you can afford to eat, drink and be merry 24/7 (perfect stag do requirements!!) I really reccommend daily foot massages at 3 quid a pop to keep your energy levels up!!
As things do change quite quickly, I would reccommend you do a search on some Bangkok websites to see what's hot at the moment.
Go there well rested and with an open mind and you'll have a ball. Someone will probably try to rip you off at some point, but on the whole, Thai people are probably the coolest on the planet. Fun loving and with a wicked sense of humour.
PS: If you need accommodation, try asiarooms.com, as they have good photos, reviews and always seem to be one of the cheapest.