Onboard RAID 5 - Intel or AMD?

Intel would work better and faster, but note that if you enable the 'write caching' option in the RAID driver utility, your filesystem could be corrupted in an unlucky unclean shutdown; like a blue screen or power outage. It also relies on your RAM more than other solutions, meaning that corruption on your RAM would translate to filesystem corruption on the disks/RAID volume.

So keep a good backup and do not consider RAID5 to protect your data; these solutions are not that reliable; i would consider them on par with a single disk without RAID; which is actually quite good, but not good enough for your precious data i would assume. Keep real backups on external disks, for example.

You should use a decent CPU but any recent dualcore would do honestly.

Agreed. I recently lost a ton of data (4+ TB) on a media server I THOUGHT was on a redundant RAID-5 array.

I've had experiences with software RAID (Linux), fakeRAID (Intel mobo) as well as hardware. All have pros and cons.

The draw on your CPU really isn't severe at all, I wouldn't worry about it. He's quite right about any dual core being able to handle that. If you are building for HTPC reasons, the processor will have more than enough power to handle soft-RAID.

Please, please, please keep in mind, RAID-5 does NOT mean "backed up". Homebrew RAID arrays drop hard drives all the time, especially the "green" drives.

That said, you never specified what TYPE of RAID you were going for, so before we assume TOO much, what did you have in mind?