Well as of right now using a dual core for FFDSHOW doesn't really improve anything since you're not dealing with a multi threaded app. I'd want to get the most horsepower for the price and as of right now dual core ain't it.
But as mentioned by INANE and Danmagic7 that is changing and soon most applications will be optimized for multicores (even consoles are doing this; think XBox 360...). Also if you're planning on having your HTPC do many tasks at the same time then dual cores would definitely help. I'm assuming you would be playing videos and nothing else at the same time.
Now if you can afford to buy a dual core then by all means do! My last build uses an Athlon 64 X2 4400 (overclocked) and I love it!
