I've experimented with fans (braought 5 different sizes to see which fits best) but even though the sound from one of them is almost silent I still can't stand any noise what so ever. Just messes the experience up (also running XBMC). Just learnt something new though if your interested. I have my Pi set up at 45 degrees from Horizontal, so its almost vertical. This improves heat dissipation 2 fold, both surfaces are exposed so less heat built up and 2/ the air is allowed to travel up on its own through the fins.
and lastly this is just what I learnt today with my setup (almost vertical) if you place another heatsink atop of the first it actually reduces heat alot more (3-5 degrees more) I first thought that it might trap it but it doesn't. I also brought like extra pieces of heatsink (came in package of 5) so they are awesome.
Thats the reason I thought it would be awesome to cover the whole area ontop of the Pi for the most efficient heat dissipation.
To be honest with you the Pi is showing its age now in regards to being an XBMC box. I won't be upgrading anything anymore and will wait a few months before arm android matures and you would have android sticks that are at least 10 times more powerful (arm) for cheaper price than a Pi...