Cards with the inbuilt XOR processor dont have to rely on the CPU for the parity calculations, so writing data will be faster and less taxing on the system.
As for linux RAID5, are you sure it supports online array expansion? I always thought you had to wipe it if you wanted to add more drives - been a while since i have looked at it though.
Linux RAID supports IDE drives too.
Finally RAID is not a backup - remember this, keep a seperate copy of all your important files
