Multitasking will be there in iPhone 4.0 and iPhone 3gs can be upgraded to it
if you jailbroke your iPhone.. iPhone becomes a phone above all.. you can hardly think of a feature it lacks after jailbreaking it..
Yxflash a app for iPhone allows you to watch 1080P videos on 3gs.. multifl0w alows the multitasking.. iBluenova for bluetooth file transfer..
3gs is one of the best iPhone get it.. don't wait for the 4g as grey market pricing will cost a bomb atleast for a year..
and when you can upgrade to 4.0 then there should be no waiting..
N79 is like a kid in terms of usability in front of iPhone