The board is a good choice IMO. Power supply should allow use of a gaming card later.
For the CPU, since this is a HTPC, you may do a bit of video transcoding...get a Quad.
Memory will be DDR2. Really anything is fine, but I recommend Corsair.
I'd suggest a larger HDD. The sweet spot on value is 1-1.5TB. For a HTPC, 500GB isn't much, especially if you have lots of HD video. Heck, the 2.5TB on my PC isn't enough as I do a lot of HDTV recording (6GB/hr).