It should let you choose a partition during start up already. It doesn't matter which one is active or not, when you boot your computer, a text screen allowing you to choose your posion (operating system) should come up and it will give you by default 30 secs to choose which operating system. This doesn't happen?
Well I have really only played with MandrakeLinux, v10, and I set that partition as active. It has a graphical boot screen that lets you: Boot linux normally, safe mode, Windows 98, Windows XP. (in my case).