So far it looks like the best 4GB DIMMs are made by Samsung, with IC HCH9 40nm/30nm.

How to distinguish 40nm vs 30nm: K4B2G0846C/D the last character is the die revision - 40nm is rev C, and 30nm is rev D.

Overclockability:

Typical 30nm:

1600 7-8-8
1866 8-9-9
2133 9-10-10

Typical 40nm:

1600 7-8-8
1866 8-10-9
2133 9-11-10

These Samsung gems have been very popular in Asia for a while, however it is a pity that it's extremely hard to buy them in the EU (or perhaps even in the US).