Unfortunately, the fakeraid doesn't really work in linux (generally speaking, there are exceptions and workarounds). Basically, unless you are gonna spend the money to get a real raid controller (probably $250+), software raid (mdadm) is your only real option. Your cpu should be fine, based on it just being a file server. Looks like you have a total of 8 SATA ports and a few IDE ports, so you should be ok on that, unless I read your post wrong.
For comparison, I have a BE-2350 (2.1 GHz x2) that runs as a home server; file/print server, plays movies (bluray rips) via HDMI to a HDTV with room to spare, DNS, DHCP and probably a few other thinks I dont remember setting up. I have 3x1TB in a raid 5, its easy to setup. Basically, follow that link and the directions are still current for any linux distro. Personally I prefer Debian, but the OS is almost irrelevant, the software is available on all. The configuration is what matters. Installing the OS on a separate drive is the way to go, keep the OS off the array. While it is possible to install it to the array, it may end up being more trouble than its worth in the long run. Backup the config files once you have them setup and if it crashes or whatever, a reinstall takes ~20 minutes, copy the files back in, and done.
For what its worth, that array was originally created in one box running Fedora. I later ditched it and installed Ubuntu. Reassembled the array, 5 minutes, no data lost. Moved the 3 drives to another box, different hardware, and installed a newer version of Ubuntu. Again, reassembled, no data loss. Later installed Debian, reassembled, you get the idea. The main OS drive has been slicked at least a dozen times, and each time the array is reassembled easily, and with no problems.
One recommendation would be to put all 8 drives as one big raid array. Raid 5 utilizing one or more as a spare would probably be my choice, but do what you like. Instead of creating 2 arrays, one for everyone and one locked down, make one, and use proper file permissions when setting up the share and you would achieve the same results. With proper Samba permissions, there is really no need to do separate arrays, it is just more trouble than it is worth.
Any linux OS can also handle that backup you mentioned, read about crontab.