RAM stats

[hypochondriac]

Ok so I know what the RAM memory size is, and i think I know what DDR2/3 etc is, but can someone breifly explain in laymans what the other bits mean like 800Hz, 5-5-5-12, and PC2-6400C4 mean please?

 

[hypochondriac]

well that clears the timings up, but what of the frequencies?
Also is performance heavily depended on these stats? ie will getting 1066MHz be a noticable improvement over 800MHz?