I agree with hartofwave that an APU would be nice for a stream box. It would keep the package small and you honestly don't need that much to be able to stream full 1080P. Actually, as I typed that, you might even be able to get away with an Intel cpu with graphics. They've got a nicer selection of ITX motherboards that come with WIFI integrated.
I don't know if you're in the EU or the USA, but here is a sweet little build someone did in the XBMC community:
It was for about 500 pounds, which is a bit more in USD, but thought it would be a good outline to recommend anyway because it is a stream box and I think a nice one at that.
If you're interested, here is my build log for a micro ITX build I recently did and will likely never be complete with. It is more oriented as ultra portable gaming though, so the SSD and the A10-5800k are completely unnecessary for what you're looking to build.