Regarding putting together a Linux box make sure to do this on an X86 system and not on a G4/G5 system. Reason is software such as Maya, XSI, Houdini, Shake, Smoke, Digital Fusion ported to Linux are coded to run on x86. Trying to run them on a distributions such as Yellow Dog on a Mac isn't going to work.
As for the discussion which OS is better has been debated on many forums. Basically it comes down to TCO and your needs. From all the benchmarks done (ie: see zoorender.com, shake benchmarks, etc) it seems highend apps as mentioned run faster and more stable on X86 hardware than on a Mac. This may be in part to having access to DCC hardware such as Quadro and FireGL where as the Apple user has been left with Geforce and Radeon cards that don't always perform as well as their big brother. Over all I've found Novell and Red Hat Linux distributions to offer better stability, security, performance and lower cost than Windows XP Professional. Nice to not have to worry about things such as spyware or viruses. The same could be probably said for OSX users when comparing against Windows TCO. In the end it's really up to you so ensure you choose what's best for you taking into consideration the software and hardware you'll need/want to use.