yes, but not all iphone 4 apps are available on the iphone 3gs/3g/2g becuase they are too high resolution apps, or need a front facing camera.
Through iTunes
you will have to jailbrake the phone ...
IF you have a computer and have installed itunes on it (no duh you wouldn't have apps with out i tunes:P) all your apps, videos and songs SHOULD be saved on i tunes so just sync the iphone 4 2 itunes and there you have it. :)
yes infact the iphone 3gs does have AIM (aol instant messenger) it works very good and to donwload it all you have to do is go to apps on the phone and type in AIM but usually it will cost no money and sometimes it will but it does tell you. to get an iphone 3gs log onto att.com and type in iphone 3gs in the search bar. and you can chose between 8gb 16 gb and 32gb.
by juggling you nuggets for a while.
if you want to record videos, then you can find tons of apps that will allow you to shoot videos.
You transfer them in iTunes under the "Photo" tab in settings.
iPhone 3gs
iPhone 3GS
the iphone 3gs replaced the iphone 3g.
Dose the iphone 3gs have a radio