So, is this HTPC just for video/media playback? Or is this a "HTPC/Gaming" machine? Because that changes things.
Basically, any AM3 CPU will work for you. The very generic rule of thumb for HTPC media playback use is you want a 2GHz or higher dual core... anything. So you could conceivably go as low as the a sempron 2 if you can unlock it. As it is, I'd go with whatever you can get a good deal on. A quick look at newegg places the sempron at $38, and the most expensive model they're still selling is the Athlon II x3 450 (3.2GHz triple core), and that one will set you back $70.
So that part stays roughly the same, no matter what you do in terms of gaming or media playback. The GPU will be much different.
For media only type stuff, then $30-40 on a 5450 or something like that should be more than enough. If you want to be able to game, I'd go with something like the 7770 (my buddy just got one from newegg for $70) or thereabouts. The 7750 is about as fast as you can get that doesn't need its own power connection, which means it should stay nice and quiet, but if you don't care about that, and your budget is $200 for both parts, you could go up to the 7850-70 or something like that.