What are the RAM requirements for OS X 10.10 Yosemite The minimum requirements for RAM for Yosemite is 2 GB as stated in apple literature.

What are the optimum real world requirements for Yosemite to achieve a noticeable improvement.

In particular for an early 2011 MBP 2.3 GHz which currently has 4 GB.
Absolute minimum is 2GB... though 4GB would be a more tenable minimum, 8GB more comfortable, 16GB would be able to breathe properly. 

8GB really is the 'minimum sensible' for a modern OS, whatever the official spec says.