A lot of new users will begin to JB their devices now that Limera1n is live and I want to ensure their success. If you install an app from Cydia that is NOT compatible with iOS 4.x you run the risk of your phone kicking into safe mode a lot. So below is a link with a spreadsheet of what actually works on iOS 4.x. Please check the list prior to downloading things to ensure you won't have issues with your new JB device. Good luck and have fun!
iOS 4.x Compatibility List
iOS 4.x Compatibility List