The main difference between DDRII & DDRIII is latency & in most cases operating speed (Bandwidth). First you should understand latency, the interval between stimulus and response. CAS-2 you wait 2 clock cycles and for CAS-3 you wait 3 clock cycles.
The lower the CAS latency (given the same clock speed), the less time it takes to fetch data from it.
CAS latency (CL) is the time (in number of clock cycles) that elapses between the memory controller (CPU FBS) telling the memory module to access a particular column in the current row, and the data from that column being read from the module's output pins. CAS latency only specifies the delay between the request and the first bit. The clock speed specifies the latency between bits.
RAM manufacturers typically list the recommended timing for their RAM as a series of four integers separated by dashes (e.g 2-2-2-6 or 3-3-3-9 or 4-4-4-12 and so on). While there are many other settings related to RAM, these four integers refer to the following settings, which are typically listed in this order: CL - TRCD - TRP - TRAS.
CL = CAS Latency time: The time it takes between a command having been sent to the memory and when it begins to reply to it. It is the time it takes between the processor asking for some data from the memory and it returning it.
TRCD = DRAM RAS# to CAS# Delay: The number of clock cycles performed between activating the Row Access Strobe and the Column Access Strobe. This parameter relates to the time it takes to access stored data.
TRP = DRAM RAS# Precharge: The amount of time between the 'precharge' command and the 'active' command. The 'precharge' command closes memory that was accessed and the 'active' command signifies that a new read/write cycle can begin.
TRAS = Active to Precharge delay: The total time that will elapse between an active state and precharge state. This is the sum of the previous timings: CL + TRCD + TRP.
The main thing to look at between 800MHz & 1333MHz, again is the latency. Technically because 1333 operates faster it goes through clock cycles quicker then 800, but because it takes longer to access data per clock cycle (latency), the 1333 may not appear to be any faster than 800. Keep in mind that typically, the lower the CAS, the higher the price with in the same operating speed.
Something to consider is trying to match the (FBS) Front Bus Speed of the (MB) Motherboard, the (MCM) Memory Control Module AKA FBS of the CPU & the Operating speed or Bandwidth of the RAM. Here is an example configuration of this suggestion;
MB –
FSB 1066/800MHz
CPU –
FSB 800MHz
RAM –
Speed DDR2 800 (PC2 6400)
Cas Latency 4
Timing 4-4-4-12