For the 'mini-SIM to micro-SIM' problem:
- Use a pair of sharp scissors or a micro-SIM cutting tool to cut the mini-SIM card into micro-SIM. (be careful though)
- OR contact your mobile network service provider and tell them you need a micro-SIM card. They should give it to you for free for the first time.
iPhone 4 and iPhone 4S use the micro-SIM card. You can take the micro-SIM from iPhone 4 and insert it into iPhone 4S without any problems. All the contacts saved on the micro-SIM will be there.
----------
For 'contacts':
(1) On your iPhone 3G, go to 'Settings' -> tap 'Mails, Contacts, Calendars' -> scroll down to Contacts and tap 'Import SIM Contacts'
(2) After step 1, sync your iPhone 3G with your Windows-PC.
Having done the above, when you get an iPhone 4 or iPhone 4S later, you can just sync it with your PC and you're good to go. No need to type all the contacts from scratch.
'iOS: How to transfer or sync content to your computer'
http://support.apple.com/kb/HT1296