While OS X can coexist with 9, I've found that when using XPostFacto, Classic and direct-boot-9 are more stable when installing X to a separate partition than 9, then installing a SECOND copy of 9 on the OS X partition to use just for Classic. (It only matters if you plan on dual-booting regularly. If you plan on using X exclusively, go ahead and stay with one partition.)
One thing to consider is the beige's 8 GB ATA limit. OS X must be installed on the first partition on a given drive, and that partition must be under 8 GB. (Use 7.7 to be safe.) This is due to a limitation in the beige's hardware, and can only be fixed by using a SCSI hard drive, or an add-in hard drive controller. (This only applies to ATA drives.) It can be on ANY ATA drive, but it must be in the first 8 GB. The installer will warn you of this.
So if you have a drive larger than 8 GB you want to install on to, you will need to partition it. (It doesn't have to be installed to the 'primary' drive, and it will recognize larger drives/partitions that aren't the boot drive, but it can only boot from a max-8-GB partition.)