Metusion, I'm new to Android and the Galaxy S and don't pretend to be uber technical but hopefully some of this will help. I would suggest if you need help then to ask specific questions (these forums really can help)
When you say your device hangs... define this? is it a lag of a second or so? this is well documented and awaiting a fix. If you are in the UK samsung have commited to 2.2 in Sept and a fix for the "lag" issue.
Wifi signal is weak.. no idea sorry can't help
Latest Firmware - always the best place to be
Apps running in the background - As stated I am new to android and multitasking was to be frank one of the attractions. Some apps are designed to run in the background, most have settings which you can adjust to stop them killing battery. Some will be set to auto-load when you turn on the phone. If you don't like a particular App running in the background then delete it.
Stability - In the past 4 weeks when I first purchased this phone I have only been a victim of instability once. Downloaded an app to control WIFI access and somehow this killed my ability to connecto to WIFI.. not good. Factory reset fixed the issue and deleted everything from the phone (thankfully calendar & contacts were already online and safe in gmail) I suspect if your device is flakey it may be due to downloading a lot a "useful" apps since you got it. might be worth backing everything up to an SD card and doing a reset to see if this makes it more stable. If it is still unstable with a factory reset then consider pushing for a replacement.
NB a reset will wipe everything so make sure all your contacts and anything else that you want to be able to restore are backed up.
Hopefully the above helps in some way. I've had iPhones since the first was launched and I would trade every one of them for my galaxy S.