I've seen this happening in other games as well, but mostly it's due to issues with the hardware itself and not generally with the game. When I first got Civ4 I had a bunch of crashes and later realized that my overclocked CPU was not as stable as it appeared to be. I lowered the overclock and the game ran MUCH smoother.
With a lot of RAM, you get the benefits of better performance, but also run the risk of a faulty RAM chip which might only rear its ugly head when under a high level of strain. CIV4 is VERY CPU and RAM intensive so a lot of times it might just be showing faulty hardware that no other software had been able to detect. This is usually the cause of the unrepeatable crashes and the 'random' crashes that are typically seen. This is common in any high CPU intensity software. Even if it's non-overclocked, standard hardware it can have faulty pieces that can really tick you off!