iPhone-How to upgrade, wipe old phone

[markw10]

I am planning on buying an iPhone 4 any day now for both me and my fiancee and currently both of us having iPhone 3G's. My plan is once I get the iPhone 4's to sell the two iPhone 3G's on ebay.
I have two questions:

I have never gone through an upgrade before. I am looking at options such as Radio Shack, AT&T (Refurbished) and Apple. When I buy one do I activate it on the spot in the store or do I get it and then have 30 days to active it? I know when I connect my iPhone to iTunes it backs up.
When I get a new phone do I connect to iTunes to activate and then restore the backup?

The second question is how to securely and completely wipe the old phone. I want to make sure since I'm selling it on ebay that no one can get data off of it and obviously I want to do this after I active and have my new phone working. What is the best way to do this?

Thanks for your help with the above questions.

 

[Cleveland]

if you go to Apple it'll be activated there and then you just plug it into your computer and it'll recognize it's a new iphone and you can restore from backup and viola upgrade as for ATT refurb or radio shack i'm not sure if they come activated or not, i don't think and once you plug into iTunes it'll activate them


As for wiping data once you know the backup restore was successful, just go to settings -> general -> reset -> erase all content and settings

and follow the prompts

hope this helps

 

[anon(4697585)]

It's all written down here Tip: How to erase all personal data from your old iPhone | TiPb

 

[kalia]

the same problem to me ,thank you!