Don't forget a PSU..Corsair, Seasonic, antec, xfx seem to be the recommended brands.
Oh yeah PSU definitely. I'd make sure to get a quality one and not a junk one.
Most people don't even think about it, but I have built a cheap NAS server that I use to back up my SteamApp folder. So when I reformat I don't have to waste time re-downloading.
Not a bad idea - you could just buy another HD and run it in RAID 0 and then have automatic backups.