It is a major difference between DDR and DDR2.
Socket AM2 (AMD's Upcoming socket) will use DDR2 RAM on it's motherboards, since DDR2 runs at faster clock speeds, there is of course a down point. The cas latency, The cas latency on DDR2 RAM is MUCH higher than DDR, to define cas latency it is basically measured in nanoseconds the amount of delay between running applications, thats why the lower the CL the faster you perform.
CAS latency is determined by dividing the column access time by the memory clock cycle time. Simply put, the CAS latency is the number of clock cycles it takes the memory to respond to an operation request. Lower CAS latency, therefore, will normally give slightly better performance than higher CAS latency. The default CAS latency for SDRAM is 3 and for DDR is 2.5. The lowest possible CAS latency is 2.
Alright now for the example between DDR and DDR2 RAM's speed compared to CL:
Notice on one of those timings it is 4-4-4-12! So I guess it's safe to assume the average DDR2 Cas latency is 4.
Now on to DDR:
Now you can see the average CL for DDR RAM is around 2.5-3 going as low as 2.
As I've shown there are some advantages to both....wow...I just rambled and spent way too much time on one post that wasn't even relevant to the topic, I feel like a high school teacher.