Dual-boot Mac OS X and Ubuntu Lucid with OS X Installed First

[dannymichel]

Was wondering how to do this. I was trying to do achieve this from looking at different guides, but I haven't had any luck. I am on a custom PC, with dual screens; one LCD, and one CRT, from which I installed OS X 10.5.6 from a retail DVD. I installed OS X first, because I needed to format my HDD to use GUID partition table, because you can't install from a retail DVD without using GUID. Once OS X was installed, I partitioned my hard-drive into 4 partitions from the OS X disk utility. These partitions were HFS. I then used the Ubuntu Lucid x64 Live disk to format the 3 nre HFS partitions to use ext4, for two of them, and one swap. I installed Ubuntu as normal. I re-booted, and GRUB recognized my OS X instillation, so I tried to boot into it. It went into OS X, but with some major problems. My LCD screen was going haywire, but my CRT seemed to be working, but it took on my LCD's screen resolution and place as the main screen. I thought my OS X instillation was badly damages, so with Ubuntu still installed on the other partitions, I re-installed OS X, which I am on now. I want to know how to boot back into Ubuntu, while still having the option to boot into OS X. Thanks in advance for any help.