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