I love my BigNG (though it's not currently installed... again...) it's a very versatile piece of kit.

I'm not sure exactly what it is you're doing, but two of them is probably overkill, and (if memory serves) can give hiccups if running them simultaneously on the system (rather than as independent units.) However, it seems likely that one BigNG and one miniNG would do you... the two 5400's on the miniNG and the rest on the BigNG

One thing to be aware of is that mCubed offer very little (no) support, if you aren't confident you can get it going then perhaps hesitate and get one of the pending new aquacomputers.