Snow Leopard includes that claims to offer many improvements over QuickTime 7. However, the new version may not support some older media formats that QuickTime 7 handled with aplomb (see for details). Check this option to install QuickTime 7 for backward compatibility. If your Mac has an active license for QuickTime Pro 7, this option will be enabled by default.
Even though you have all these options to consider, don't spend too much time worrying about what to install: unlike Leopard and earlier OS versions, Snow Leopard can automatically download and install some software on demand. For example, if you didn't chose the All Available Printers option and you later try to use a printer that's not supported by your original installation of drivers, OS X will connect to the Internet and download drivers as needed. Similarly, if you didn't install Rosetta or QuickTime 7, and you later try to open a program or media file that requires that software, OS X will offer to download and install it on the fly (assuming you have an Internet connection, of course).
In addition, if you forget to install something the first time, or if you're having problems with an application and you want to get a fresh copy, you can easily install it from the Snow Leopard DVD by double-clicking on the Optional Installs.mpkg package inside the Optional Installs folder. After the installer launches and checks your drive for Snow Leopard, you can choose to install Rosetta, QuickTime 7, additional fonts, printer drivers, language support, iPod support files, X11, or any of the stock OS X apps (Address Book, iCal, iChat, iTunes, Mail, Dictionary, or Safari).
After choosing your options, click OK and then click Install, verify that you want to install, and enter an admin-level username and password. The installation will begin immediately--the computer doesn't need to restart first. If you're using a Bluetooth mouse or keyboard, you may have to re-pair it at some point, but otherwise, the next interaction you have with your Mac should be when it reboots with Snow Leopard successfully installed.
If you've set up your Mac to be able to boot into Windows via Boot Camp, you'll need to update the Windows drivers using the Snow Leopard DVD. Be sure to boot into Windows, insert the Snow Leopard DVD, and follow the instructions to update those drivers.