Corsair Hydro H55 on mATX motherboard?

Yes, it will fit. You might have to mount the fan and radiator on the sidepanel unless the case has a mount for 120mm man in front, then I'd mount it there. I cant see that from the pictures I have of the case if there is a 120mm fan mount there. But when you mount the H55 be sure that the case has positive pressure inside. (More airflow going in than out.) BUT with at least 1 fan blowing the hot air out.

EDIT: I see that there is a 120mm fan slot in the back. Yes it will fit there I believe. And if you do fit it there, have another fan blowing from the sidepanel onto the CPU/Northbridge area to keep them cool.
 


Thanks for the help! :)
Just one question, if the fan on the H55 radiator is mounted as intake, to pull cold air inside the case, then the side mounted fan should be configured as an exhaust, right? Or do you recommend I mount the radiator fan as exhaust and have the side mounted fan as an intake?

 
NP =)
Yes, that is how I would do it. The fan on the H55 can be mounted either way. Also as a pull fan.
|case|<---fan|radiator|
but i'd advice something like this:
|case|radiator|<---fan|
Or if you have enough space and fans -push/pull:
|case|<---fan|radiator|<---fan|
But you can have it to blow as an intake if it fits better. In that case have the side panel fan acting as an exhaust. hen it would look like this:
|case|fan--->|radiator|
or as an example with too little room in the case with push/pull (also works reversed):
|fan--->|case|radiator|fan--->|

In any case have more air going in, as I said before, than out. And, yes have a fan blowing fresh air on the CPU/Northbridge area.

Good luck =)