How do you backup all my apps in the iPhone to PC?
itunes will back up your apps when you sync your iPhone if the computer is authorized (store-authorize computer in itunes).
How do you transfer apps from itunes to iphone 4 without losing other apps on allready on iphone?
eBay would be your best bet
Can you buy apps from apps store on your iPhone 3g when unlocked and on tmobile?
I would believe so, it can be difficult to unlock your iphone though but the iphone is exclusively at tmobile in Europe.
Can you get apps from the apps store after jailbreaking your iPhone 3G?
you can still get it sometimes but sometimes the app store dose not work for example on my ipod it works but on my iphone it does not work
Does it cost money to run apps besides for buying the apps in the first place on the iPhone 3GS?
no, once the app is purchased ,to play it or run it doesn't cost money :) hope this helps,lex