How frequent are DRAM errors?

How frequent are DRAM errors? How significant a factor is heat? Is it worth investing in ECC RAM?

A paper addressing this issue was published here.

"The goal of this paper is to answer questions such as the following: How common are memory errors in practice? What are their statistical properties? How are they affected by external factors, such as temperature and utilization, and by chip-specific factors, such as chip density, memory technology and DIMM age?"

And this is a part from summaries:

"This either indicates that chip size does not play a dominant role in influencing CEs or >there are other, stronger confounders in our data that we did not control for." >Similarly, "In all cases, for the same utilization levels the error rates for high versus >low temperature are very similar."

I would still recommend ECC for production systems, mind you.