Memory !! CL9 CL10...

CL means cas latency.

Heres a good guide to understand about ram


with ram, the size is the first major factor(i.e comparing 1333 to 2166 ram, the 2166 would be faster) if the ram are of the same speed, i.e 1600 vs 1600, then the one with the lower cl would normally be faster. i.e 7-7-7-7 is faster than 8-8-8-8

lower latency mean faster(CL9 faster than CL10)

IS THERE ANY 7-7-7-7 IN THE MARKET ! BUT I THOUGHT CL9 IS THE FASTEST MEMORY  
CAN U REC ME ANY 2X4GB BUS2000  

high memory band width is just a little bit faster ,2x4GB 1600 CL9 is perfect
invest your money on a SSD or something else

HOW MUCH FASTER?
BUT IVE HEARD 1600 WILL WORK ON 1333 AS DEFAULT  
IS 1866 GOOD CHOICE?

it depends on the mobo and cpu. i know that sandy bridge/ivy bridge will bring 1600 down to 1333, unless you change in in bios in a few builds. some AMD processors dislike having large amounts of high sized ram and lowers it when it reaches x amount of gigs as well.

2400 ram is just 5% faster than 1600
expensive RAM dont worth the extra money
SNB work on 1333 on default but if you have a Zseries(z77/z68) motherboard or Pseries(p55-p45-p67) you can overclock it to 1600(you cant make over clocking if you have Hseries mother board
ivy bridge processors work on default at 1600