RAID has nothing to do with "backup". They are orthogonal concepts.

RAID, except RAID 0, is about *availability* not backup. It makes sure that the the system can be used, and that you can access the data, whether the data is good or bad.

If the editor scrambles your video, RAID ensures you can get to the scrambled video all the time. Backup lets you access an older version of the video before it was scrambled, but it might take an hour or two.

Quit worrying about the brand of CPU and whatnot. That's putting the cart before the horse. Your decision is apparently between Final Cut and Vegas. If one choice requires more expensive hardware then it's a negative factor for that choice, but you are still choosing between Final Cut vs. Vegas and not the hardware.

An iMac is built with notebook components internally. This limits performance and makes them a bit more expensive than a similar PC. The Mac Pro uses desktop / workstation components.