Moving Sim Card from One iPhone to Another I am looking to buy an iPhone 4 from the secondary market. I currently have an iPhone 3GS. Can I just replace the sim on the new iphone with my current sim? Or is there something else that I need to do?
If you want to make your SIM into a compatible iPhone 4 Micro-SIM, you'll need a micro sim cutter. Look these up on ebay in your area. They're as cheap as chips and save you a ton of bother.

Look for DIY ways of making pre-iPhone 4 SIMs into iPhone 4 Micro-SIMs on YouTube. There's video instruction for how to do it yourself