Alternatively, you can buy the old Mac Mini and swap out the slot-loading drive if you feel so inclined. It still has a GeForce 320M as GPU and a pretty competent Core 2 Duo.
But sincerely, the only reason why I think the Mac Mini is "almost" worth the amount Apple is asking for this generation is because Intel has significantly boosted their mobile CPU performance while leaving desktop chips out to dry.
That basically means that whatever you put together to build an HTPC to match the Mac Mini, you'll just hit about the same price point and you still have to build it all yourself, but you'll never match the same power consumption or overall package size. It's like Intel is playing favorite to Apple this round, knowing that Apple mostly use mobile chips in their computers.