12Gb of 1800Mhz ram will only be faster than 6gb if a lot of ram is actualy being used, otherwise the 2000Mhz ram will have a lower latency and be quicker.
Also, the ram will have trouble running at that speed with all banks full, unless you do 3x4Gb which is very expensive. Filling all banks puts more stess on the IMC and can mean a bigger limit on the overclock of the ram and cpu. To be honest, in real life there is virtually no difference between 1333Mhz up to 2000Mhz as triple channel provides well over 20Gb/s anyway, I get over 30Gb/s with my ram at 1740Mhz (stock 1600Mhz) and socket 775 only has a bandwidth of up to around 10-12Gb/s.