The HD3450 is plenty fast for pretty much anything you'll do with a HTPC. I wouldn't expect any sort of gaming performance out of it, but for HT work, it's value is unbeatable.
If your just looking to cover HD and not game, then the 3450 is enough. The 3470 is the same as the 3450 but it has DDR3 instead of DDR2. The 2400 is about the same as the 3450 but consumes more power and does not have powerplay(power consumption feature). The 8500 is along the same line in performance but only decodes 1 kind of HD format, ATI does two, and I believe it consumes more power then either the 3450 or 2400.
If your not looking to do HD then just about anything cheap will work.
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