I Think We're in the Black.........
The WD "Black" edition drives are intended to be roughly equivalent to Seagate's ""ES" line of Barracuda Drives, "ES" meaning "enterprise service". So, they're a step up from the "green" models.
It's probably just superstition on my part, but if I were going to set up any sort of RAID array, I wouldn't want to contend with a drive that throttles up and down on it's own internal whims, such as the WD "Green" models do. 7200 RPM and steady as she goes, that's what I would find comfort in.
WD's drive suffix letters "SE" stand for Standard edition. There are also "RE" or "RAID Edition drives such as this;
http://www.newegg.com/Product/Product.aspx?Item=N82E16822136313 The price on these is a bit over the top.
At one time WD's smaller capacity "SE" drives didn't support "NCQ" or "native command queuing". Hence you had to spring for the "RE" models, but I think that at 400GB and above they all do.
Intel's "Matrix Storage Technology" drivers require NCQ, (when last I checked), as to whether other RAID solutions do, I can't say.
To answer your "how hot is hot" question, part of the answer resides in how much airflow your case can provide. A cramped case with no fans is not what you're looking for.
While we're on the subject of hype, since the average HDD draws less than 20 Watts, how much energy saving do we think we're going to get out of green vs. black? That's sort of directed at WD, not you.