The question of fitting the Ubuntu ver 8.10 on a CD has been answered already.
I don't have a lot of experience with Linux OSs, but if you intend to operate a OS from a CD copy of a download, it will likely be in a compressed form. So, if it is compressed, it will perform slower than it normally would if an install was done (just so you know it wouldn't be slow if installed). If you can, or know someone who can burn the files you need to a CD in their uncompressed form, that type of CD will operate better than running the compressed OS which includes a lot of data that you probably don't need.
If you can boot from an external source depends on your motherboard capabilities, Joe.
You would need to look up the motherboard's capabilities at the manufacturer's website, and/or look in the BIOS settings of that motherboard to find out for sure.
Another thing the manufacturer's website can inform you of, is whether there is a BIOS flash update for that board, which may add some capabilities that it may not have, if it has not been updated. If there is an update, and you choose to proceed, you'll need to follow the provided instructions closely, or bad voodoo can take place.