The key to getting it install right is putting X on the first partition. I have 6 GB on my first partition and 5 GB on my second. I installed 9.1 first onto the second partition by clean installing 9.1 from the 9.1 CD provided. Then I installed X to my first partition. You need at least 128MB of RAM, 192MB of RAM recommended according to Outpost.
RAM is cheap now.
I have 192 MB of RAM, and I did a clean install of OS 9.1. 1.5 is the minimum. The next partition down from the 1.5 GB one is 3 GB, has OS 9.1 installed, and plenty of room. I was excited as can be to get my new PowerBook this week. My excitement has ceased and I am trying to convince myself that going back to Mac was a good thing.