Hey,
I am on a Mac Pro dual booting right now with OS X and Windows XP. I also have an external HD that is running OS X 10.5. Lastly, I have a copy of Parallels. So my questions areThanks:
- What build of Linux should I use if I am a newbie?
- What type (x86, 64, etc)
- Where should I install it (I am happy to re-partition my external HD)
Thanks!

I've had Rock and Ubuntu running on my Macs without any problem. I've typically toasted the Mac OS, but there shouldn't be any issue in creating a multiboot. You can find more information out via distrowatch.com or google these two OSs. I've never tried a triple boot with OSX ... might be interesting to ponder as a later project. Which distro you choose will depend upon what processor you have. The beauty of Ubuntu is that it has distros that work across both Mac and PC so using it on machines as a cross platform make it very practical (least in my house ... my wife can use either without freaking out).

Well, I can triple boot, I have some code for it. My system specs:
2 2.66 Ghz Intel Xeon Processors
1 GB Ram
500 GB HD
256 MB Video card

So, if I make a bootable Ubuntu DVD, what build do I use?

I prefer to use grub as my bootloader so I install Ubuntu last. During the install it will detect your other OSs (provided you don't tell it to do a clean install, overwriting them). I found it to be a piece of cake to do the multiboot - install Windows to the primary partition, and then your linux distros to their own individual partitions. I also allow Ubuntu to install Grub into the primary boot partition - some folks will tell you it'll cause problems, but I've never experienced any issues at all. There are plenty of tutorials on how to re-write the config file to change the boot order, splash screen, etc.