the on board raid controllers on every board ever made a joke. They are extremely UNRELIABLE and SLOW. They are "hybrid" hardware-software solution that absolutely suck. and btw, clearing the cmos will make you lose the array (but there are tricks to gaining it back; still a pain in the behind)
you should either get a quality hardware controller (300+$... the 50$ "raid controller cards" are just as bad or worse then the onboard ones) or you should do it as pure software (in the OS... windows does not support it though; only windows home server or any open source OS)
If data safety is important enough for you to bother then you should make a NAS box...
I recommend running open solaris with ZFS, shared via CIFS to windows machines.
download osol:
www.genunix.org
instructions on setting up cifs:
http://wiki.genunix.org/wiki/index.php/Getting_Started_With_the_Solaris_CIFS_Service
However, if you want it as easy as possible (and learning solaris is somewhat of a chore) you can use
http://freenas.org/freenas
Any old dual core will do as long as its 64bit (although, if you use deduplication a somewhat newer CPU will help)
the biggest improvements to speed I have seen were:
1. putting a gigE switch between the computers and the router (the router was gigE as well... and a very high end one. But it just seems that switches are plain better)
2. enabling jumbo frames (that was HUGE improvement in speed)
Oh yea... don't use raid5 or 6 even on ZFS... use individual SEPARATE RAID1 arrays. its not that much more expensive. you can start with one raid1 array, then get a second, then a third.
when you are ready to upgrade, you can upgrade them one by one. and performance will be great. (OS based raid5 is still slow... just tons more reliably, portable, and not AS slow)... using a high end card you can get it to be much faster then a single drive even on raid5 though