I actually only care about the temp of my processor, so I use Motherboard Monitor to monitor all my temp probes on my motherboard. The Hard Drives themselves are going to be warm, but it shouldn't cause problems. I just would give them enough room to breath. I have had my drives stacked one on top of the other and they get hot, but if I spread them out they are fairly warm to the touch. Then again I have two server tower cases, so I can do it that way.
I would just make sure you have equal intake as exahust. Other then that it should be fine. If you don't have to many places to put a fan then I would just go with the hard drive cooler for piece of mind. Check out <A HREF="http://www.plycon.com" target="_new"><font color=blue>Plycon</font color=blue></A>, <A HREF="http://www.pcmods.com" target="_new"><font color=blue>PCMods</font color=blue></A>, and <A HREF="http://www.frozencpu.com" target="_new"><font color=blue>Frozen CPU</font color=blue></A> and you might find a decent cooler there.