Commonly used Error Correcting Codes
I'm sure this greatly varies according to the application and the specific code in use.

Though not for DRAMs, maybe this will give you some insights:


  For CDs encoding, a (28,24)-Reed Solomon code is used.
  For DVDs, it is a (208,194)-Reed Solomon code.


Symbols are over $GF(2^8)$, that is, 8 bits per symbol.  The notation is $(n,k)$-ReedSolomon for a linear $[n,k,d]$ code ($k$ being the dimension). RS is an MDS code and has $d=n-k+1$.

(I think this was the source, but maybe you can find it online):
Stephan Wicker and Vijay Bhargava (eds.), Reed-Solomon Codes and their Applications, IEEE Press, 1994