XP Home won't use all 4 CPUs/cores on that MacPro, it's limited to 2 CPUs/cores.

As for rendercows, if I'm not mistaken you can even install cows on some of the faster Windows machines in your network and have them work together with the Mac(s). 

Intel based Macs are (probably, I don't have the data) fine and fast render machines. G5 based Macs are SLOW when it comes to Vue rendering. I've seen stats where a dual G5 2.2 was slower than a P4 2.4. 

64bit is great. That MacPro looks sweet, but make sure it has enough memory. 3 Gig looks like quite a lot, but with 64bit you'll want more. Especially if you're working with complicated scenes with lots of objects, lights, hires textures and polygons.

By the way, XP Pro 64bit isn't all that expensive. I bought an OEM version for 140 euros. XP Home is cheaper, but not by much. And it's far more limited.

The pen is mightier than the sword. But if you literally want to have some impact, use a typewriter

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