In addition to the new SDK, a new USB driver that supports Nexus One is also available today through the SDK Manager. The USB driver page contains more information.
Android Developers Blog: Android 2.1 SDKYou can refer to my article to know how to Install Android SDK on Eclipse 3.5 Galileo, in Ubuntu 9.10. Or, if you have install Android 1.6 SDK or later version, you can refer to another article to upgrade Android SDK using Android SDK and AVD Manager.