Lower timings means you have better overclocking ability.
Say a 4-4-4-12 DDR2 800MHz RAM, that means it runs at 800MHz with a timing of 4-4-4-12. So if you think you are going to run your system with a 400MHz FSB, then this will be a good RAM for you. If you think you are going to run your system with over 400MHz FSB, then its the best RAM for you also. Because you can run this RAM at 1000MHz 5-5-5-15 latency.
So...... now say you have a RAM with 800MHz but latency is 5-5-5-15.
This RAM will perform at 800MHz but if you try to overclock it to say 1000MHz which you need if you are going to run a 500MHz FSB, then you will have to loosen the timings to say.... 6-6-6-18. Unless the RAM will run at 1000MHz at 5-5-5-15. You'll know if you get the BSOD it will not run at that timings but may if you increase the voltage up a bit.
So, as far as CAS Latency goes, its basically how many times the RAM sends signals to your CPU and Mobo.
The lower the better. It's like a ping on an internet connection, the lower the ping, the better.
CAS latency - Wikipedia, the free encyclopedia
It is better to keep your system with a 1:1 FSB:RAM Frequency Ratio.
So if you dont think you are going to overclock your system at all or going to overclock it but will not go over a certain FSB, then get the RAM with the right timing.
With my system right now, I dont run my FSB to over 400MHz but I run my RAM at 1000MHz anyway with a 5-5-5-15 timing.
But if I get a 4GB (2GB x 2) G.Skill RAM with a 5-5-5-15 timing, I will be forced to run it at a 1:1 ratio and a 5-5-5-15 timing over 4-4-4-12. Unless I get the HZ's which will overclock better.