Scott: What Mac are you dumping, I just got a dual 1.25 GHz G-4 with a 20" Apple Cinama Display and it runs Carrara like a dream. For spot rendering it is almost instantaneous. Although I do understand that for what I paid, you could get a top of the line PC with a faster Chip and Bus. It came with a Radeon 9000 Pro graphics card. I use Adobe y Macromedia stuff heavily and relay on ColorSync work flow so to me a PC was out of the question. Besides, I really dig OS X. I just got it last week so I haven't tried it on any thing to complex with C. But from my initial test it runs more than adequate. It dose seem to take advantage of the symmetric multiprocessing, in C and OS X.
I my self have been thinking to get a PC, but the ones at work really discourage me, since they seem so hard to trouble shoot and unstable. But I've heard good thing about XP.