You can't.
Apple invented the iPhone 3G.
If you unlocked your iPhone it is easy to access apps store all you need is a account to download thinks from there,if you jail braked your iPhone you may download thinks through cydia only .
No, you can not upgrade your iPhone 3G memory.
Apple
No it is not as sliim as iphone 3g
no.
Yes, you can now buy the iPhone 4S unlocked direct from the Apple website
Yeah its very good I am using Apple Iphone 3G with very happlily :)
It is okay to jailbreak your iPhone 3g. The only downside to jailbreaking your iPhone is that you will loose your Apple warranty.
how to download Apple 4.2?
You can visit apple.com or visit a local apple store for unlocked iPhone.