^ not a bad setup. i too once considered building a RAID-5 system. normally i wouldn't recommend any sort of integrated RAID, but the intel server boards tend to do fairly well. however, judging by your current hard drive setup, you may want to reconsider. for RAID-5/6, your disks must be the same size, so it's fine if you have a RAID card with 8x ports or more, since you can simply add-on extra disks until you fill your ports up. but since motherboard onboard-RAID chips tend not to have much more than 4x or 6x ports, once you fill those up you have no choice but to replace all the drives with larger ones. you may be better off getting a device that supports RAID-X (instead of -5 or -6), simply because it'll allow you to combine drives of different sizes together into a single redundant array (and not just JBOD).
im going to go with RAID-X (when i get the money), which offers single-disk redundancy (same as RAID-5) but with flexibility of upgrading one drive at a time (while still increasing accessible storage space). specifically i'm looking at the
Infrant ReadyNAS NV+. it's not as speedy as a native internal RAID-5, but it gets the job done--and at a price competitive to good RAID cards by themselves. plus the software is actively developed and there are at least a dozen plugins (streaming your files, etc).