I have one 603 based Mac, and a 7200 (which actually uses the PowerPC 601 chip, which has very limited memory addressing compared to the 603 based models. Apple for the longest time said the 7200 couldn't even be upgraded to a faster cpu. A 3rd party accelerator company changed it. It is really not upgradeable very easily to 603 processors, and it wasn't an OS X candidate because of this..
The 603 model I have has one PCI slot. I have installed BeOS 4.0 and 5.0 on this . Beos 4 is very supported, but I own a copy of BeOS Pro 5, and it's PowerPC support was buggy at best.. I think they spent very little time with the update to 5.0 for PPC users.
Installing from the same CD to intel boxes you find a better supported OS with a lot more features. I hope whoever is doing openBeOS for the PPC is really revamping it, because I considered pro 5 to be lacking most of the features on the PPC version that were present with the intel version.
With 603 mac's also you have to still keep a mac OS partition on the machine even with BeOS installed and so the machine can boot. So the "open firmware" you talk about probably just mean's it can boot a non MacOS product.
