I have Arch Linux installed on my computer currently with the GRUB bootloader. Is it possible to install OSx86 (XxX 10.5.6 distro) on a partition that I prepare in gparted prior to installation, and do so without the MBR and GRUB being messed up?

Also, is it even possible to partition an OSX partition in gparted? What filesystem do I make the partition?

Yes it is possible, look at this : Triple boot : Mac OSX Léopard (iAtkos 5i), Linux et Vista sur un PC | Vrac-it.fr (french)
But you can't prepare a partition in gparted as OSX needs HFS+ partitions
Just create a fat32 partition and format it with OSX installation dvd into HFS+

That's exactly what I was looking for. After a run through Google Translate, it is readable. Thanks!

EDIT: In fact, I was able to create an HFS+ partition through gparted. I will see how this turns out when I install OSx86.

EDIT2: It didn't work. I had to reformat the partition from the CD. Oh well. I was about to finish installation but my DVD drive crapped out. >_>