Temperature IS MOST DEFINITELY a concern for hard disks. Google's study found that drives which are run at very LOW temperatures have a higher premature fail rate - there were a higher percentage of early failures, but the average life expectancy of the remaining drives run at those temperatures was the same.
But drives which are run at very HIGH temperatures DO have a shorter life expectancy - generally the higher the temperature the shorter the lifetime.
The optimal temperature seems to be around 40C. If your drives are more than perhaps 5C above or below this, then it's probably worth thinking about how to get them closer to that temperature.
45-50C should be fine. It's a bit on the warm side, but well within the acceptable range.
Hmm, well the side of the PC case is always open, so its dosn't really pack up on heat. Although the hard disks are right on top of each other, so I'm taking it pretty much heats one another up. And I can't move them elsewhere, or add a fan to it as there is no space. I'm going to put my 200MM desktop fan to the side of the case, where the HDD's are to see what it will fall too.