XBMC with wireless speakers and tablet remote

I'm a newby to XBMC and media center setup in general. I'm trying to put together my ideal system, but I need some help. Here's what I'd like to do:

- Buy a ready made HTPC, for example one from ASRock, and install Linux and XBMC on it.
- Connect an external harddrive with media files to it via USB.
- Connect it to my TV set with an HDMI cable.
- Install wireless speakers in another room
- Stream movies to my TV (a samsung HDTV) or music to my speakers
- Install a remote app on my iPad 2.
- Control all of it from my tablet.

I want to be able to play movies and music either from my harddrive or from a streaming service like grooveshark.

So, the music has to go over wifi, because I don't want cables running through my house. The speakers will be fixed to the wall or ceiling and I don't want to mock about with batteries, so they can be plugged in a socket (actually, I have a light point in the ceiling I'm not using, and I'd love to use that so I can turn the speakers on and off with a light switch).

I've seen lots of posts that make me think that the common thing to do is to stream directly from your ipad. I don't want to do that because, simply, I don't have my media on my ipad and I don't want them there either.

Does anyone have experience with this kind of setup? Can you recommend hardware for the speakers and tell me what protocol to use (can XBMC stream to Airplay speakers?)