Re: HTPC Build
I'd look at the celeron wolfdales or sempron LE, something that uses low power, or, possible, an atom + ion. Its all up to you, but the machine you build for $500 could add $30 a month to your electric bill, which is a cost that's good to consider.
I recently bought an Cooler Master Elite 360 Slim, which was marketed as a budget HTPC case. Got it for $25 at Fry's a while back. Its bigger than almost all htpc cases I've seen. Its only marketed that way I think because its designed to be horizontal. ITs nice, though. 2 cd drive slots, 2 3.5" slots (one external), and it takes full size PSUs and ATX motherboards.
Antec 300 Illusion
430w Antec Earthwatts
AMD Q4 640 Propus
2x4gb Patriot DDR3 1333
750gb Seagate 7200.11 Hard Drive