There isn't a huge amount of room inside
I'm using the stock cooler and its really noisey... something like the ACF 7 low profile
http://www.overclockers.co.uk/showproduct.php?prodid=HS-033-AR&groupid=701&catid=57&subcat=1395 should do (note I haven't tested this one specifically but I think theres room*).
These are neat little systems I love the way you can remove a couple of screws and unfold everything out for easy maintenance... but space is tight especially GPU and CPU areas.
* Tho it might be a pain removing the motherboard to fit the backplate for bolt through coolers