By your calculations, the average hard drive should last over 41 years (500/12) and a DIMM module would last over 82 years. (1000/12). Considering that the average life of a server is around 4 or 5 years, I don't see either of these numbers you supplied as "failure prone" In my experience, and believe many others will agree, the first parts that fail are mechanical i.e. Hard drives and fans. A major cause for component failure, other that mechanical, is generally heat (often times because of a failed fan...) This is just a theory, but processors are the focus of the cooling systems in a computer, as it the power supply. Often there is a heat sink on the north bridge chip and video cards as well. These cooling systems keep component at a relatively regulated temp when operating. RAM on the other hand has no cooling system, now generally RAM does not run that hot but repeated hot/cold cycles can cause material fatigue on just about anything. Still if your average DIMM will last 82 years, according to the numbers you have supplied, that is probably the reason companies are not too concerned with putting another noise making fan in the box. I have found that most problems with RAM are caused by poor handling, or heat due to dust build up. If you keep them cool, keep them clean, and supply them with clean power, your server hardware failure rate will be extremely low.
I'll have to agree with Flame here. I rarely see RAM, or any other electrical component fail. However I have seen hard drives fail on a regular basis. I've probably worked with a couple of thousand servers over the years I other than RAM that was shipped from the vendor bad I can probably count the sticks on RAM on one hand which have failed after being in use for more than 30 days.