2nding the
Wireless 360 Controller/Dongle. The dongle controls up to 4 controllers, so it's damn handy for emulating multiplayer games.
Steam Big Picture is actually pretty damn snazzy. You'll notice the majority of the games that support 360 controllers only list "partial support", but this is 99% of the time because of menus or certain overlay-load screens that require a mouse. A
Logitech K400 is the best damn thing you can buy for an HTPC for this purpose. (Or, period. Every HTPC should own one.)
If you have a receiver made within the last 8 years, any modern (read: 2xx-7xx Geforce or 3xxx-7xxx Radeon) video card outputs 5.1/7.1 surround through the HDMI, so hooking it up like you would any other device should give you proper surround. If you built the PC with a motherboard made within the last 5 years, it will also have a toslink optical out and a coaxial S/PDIF out, if those are more your bag. Typically no aftermarket soundcard is necessary.
Check out MemEx's
Mini ITX Case page for a variety of cases typically used for HTPCs, though keep in mind mITX builds aren't built to be tinkered with often, as they're layered and tightly packed. Silent parts, low-profile heatsinks, flat/jointed cables, and a focus on cooling are key.
Building an HTPC is super fun, and can be done for a reasonable amount of money. Keep in mind most mITX cases only handle videocards that do not deviate from the 2 slot width (and up to a certain length, no DirectCUII TOPs, and no EVGA Classifieds for example), so if you are buying second hand, make sure the cooler will fit.