2G to 3G upgrade

I have a 2G iPhone right now, but I'm looking to upgrade. To have enough cash to upgrade I'm going to have to sell my 2G first - probably. If I just take out the SIM card, will everything be cleared? Or do I need to use the factory reset or clear or whatever?

When I then go to the Apple Store to buy a 3G, can I just give them my SIM and have them activate it right there and I'm good to go?

Thanks to whoever can help.

You're going to have to do a restore to wipe everything from the phone.

You actually don't need to take out your SIM card to be able to upgrade to the 3G. I believe Apple has restarted the at home activation, so you just buy the phone, bring it home, connect it to your computer and activate the same way you did with your first iPhone. No need to swap SIM cards, just let it do its thing. A few minutes later your phone will be activated and it'll start downloading all the backed up information from iTunes. You'll essentially pick up right where you left off.

I would suggest wiping your first iPhone after you have successfully activated your new one just in case something goes wrong. You wouldn't want to be without a phone if you can help it.