Simply put, an HTPC is just a PC connected to a monitor/TV/HDTV that is used primarily for media playback. Everything else is optional: 10ft display (like Windows Media Center), IR remotes, and the like are not difficult to configure.
I can't comment on the case you mention, never heard of it. Looks ok, but I'm not a huge fan of Imon displays/IR combos.
Not sure what your audio setup for your HTPC would be, but if you have a reciever and 5.1 (or better) speaker setup, unless your mobo also has Dolby Digital Live or DTS Connect built-in, you will likely need to use AC3Filter or a codec pack with it built in, to configure your optical output to output in anything better than Digital 2.0 (stereo).
Win7 (at least any version that includes Windows Media Center) has everything you need for an HTPC to be effective, except for more complete codecs.
I'm not thrilled about your HDDs, but if that's all you have I'd use the Seagate and 1TB maxtor and ditch the other two. Likely to be more noise than they are worth in storage.
HTPC: OrigenAE X11|Gigabyte GA-MA785GPMT-UD2H|Phenom II x3 740BE w/AC Freezer 7|150GB Velociraptor|Corsair VX450
Main: Antec 300 (SlipStream @ 800rpm/140mm @ 5v)|Asus M4A88TD-M|Phenom II x4 945 (Mugen2 pass.)|Asus EAH6850|Samsung 830 128GB|Antec TP750
WHS: DF-85|P8H67-M Pro|I5-3450S/Hyper 212+|Corsair AX650|Sandisk Extreme 240GB, 2xWD20EARS, 2x WD15EARS, WD15EADS