The Windows 7 Release Candidate is designed to install the Ultimate edition of the operating system, even though Microsoft will probably release as many versions of Win7 as there were of Windows Vista. If you'd like, you can "unlock" the other versions of the operating system, which are hidden on the ISO you just downloaded. ZDNet's Ed Bott points out that to do so, you simply open the Sources folder, find the file Ei.cfg, and delete it.
Once you've fired up the installer, it asks whether you'd like to go online to download any updates, and then it goes and actually finds the updates—what a shock! Vista asks the same question—as did XP, if memory serves—but it never actually seems to download anything. Accept the license agreement and we're ready to rock.
The installer will reboot once or twice and ask you for information about your network, but the process is more or less unattended. Once you've finished, you're free to configure your desktop, set up your libraries, play songs anywhere with Play To, pin apps and documents to your jumplists, and explore all the other cool new features. Enjoy!