I've been considering the same. From a review I read, they used a P4 HT 2.4GHz (800MHz FSB) and overclocked it to ~3.0GHz (1GHz FSB) using the stock cooling, and it was stable.
Seeing the potential of the 2.4, I'd go with one of those and if need be - overclock it. The Bios supports overclocking of the fsb in 1mhz intervals, up to 255mhz (x4) I believe.
If you want to go nutz I've seen one watercooled shuttle box, very impressive.
The sb51g2 supports both ata and serial ata so either drive would work.
As for the bezel, you can removed the front of your drive and spray paint it, or just get a black bezel as black on silver has a cool look (that's what I'm doing, it will go well with my black and silver logitec wireless keyboard/mouse, black/silver samsung lcd, and black/silver toshiba 2000 lappy

)
I'll eventually make it into a DVR with an ati all-in-wonder, but for now I'll settle with the gf4 I already have.
For the PCI slot I'm debating my tv-wonder or audigy (I'll have to hear the onboard surround sound first)