I run Vista64 on my MacPro, and it works fabulous. Much easier than building a machine and not that much more $$$ either. Obviously don’t buy crap like RAM or HD’s or vid cards from apple though. Plus an added bonus is having a MacOS machine on site.
A fast RAID isn’t going to help you with AE or C4D as much as a boatload of RAM.
I’d say the most important thing is multiple cores (at least 8) and 2 GB of RAM per core. At least for AE, not sure about C4D.
You could always look into a render farm also. 6 old school servers is still faster than my machine.