Can I get an iPhone 4 and use SIM card from iPhone 3GS?

I got a 3GS last summer and I absolutely dislike it. I want to buy an iPhone 4 but, I'm on a 2 year contract. So I was wondering if I can buy an iPhone 4 on Craigslist/eBay etc. And just put my sim card in it? And if I can do this, can I restore my 3GS and just sell it?

Yes! You will have to buy a micro sim cutter for like 5 bucks on ebay, and it will work perfectly! Here is an iPhone 4 to try: http://cgi.ebay.com/...500wt_1156