Number of factors to consider in your case, no pun intended

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I second Davesimmon's recommendation of the 620. Its a very nice and sleek looking case that will accept a mATX board. If your going for an athlon system, you could get a mATX nforce2 board for about $100. Manufacturers include Shuttle, Chaintech and Biostar, although I would opt for the Shuttle.
Next you have to consider noise, which you want to keep at a minimium. Besides choosing the right Heatsink/fan combo you have to make be careful for what you choose as your DVD player, HD and PS. For HD Seagates are probably the way to go (I have a Maxtor which is probably the loudest component in my case - really regret getting it). For DVD players, Newegg has the Sony DDU1621 which besides being a good performer is pretty quite. Antec makes some great powersupplies that can be had for about $50, recommend the truepower series.
Then you have to consider a soundcard and a videocard. If your mostly into playing games, you could go for the standard SB route. If audio quality is more important, either go for a SantaCruz or the new Revolution from M-Audio which has been getting good reviews for its audio fidility. Finally a video card... guess your choice depends on your application and to what its going to be setup with. The ATi AIW series is a good route because it integrates so many features into its card. You could get a 8500DV for about $150.
Anyway, good luck putting together your HTPC and let us know how it goes!