Go to newegg's website and do some searching. There are plenty of good NAS solutions out there. They're typically expensive. An external USB drive might be a better solution if you're just having one PC. Also, if you're going NAS, I'd suggest using a wired connection to your router and HTPC. Things play much better and moving files around is a snap.
Just remember RAID isn't backup. If you're not keeping the discs, then you really need RAID plus a backup for the RAID.