First is indeed CAS latency, which is the most important of all settings (the lower the better)
Last is not so important, AMD systems like it high , say 10 or 11, Intel likes it low, say 5 or 6...number has something to do with correcting inbalances in your memory or something
Numbers in the middle are "Row Precharge Time" and "Read To Write Delay"
the lower the better...
If u are going to buy memory, keep in mind that the higher the memory speed (400mhz, 500mhz, or PC3200, PC4000), the "looser" the timings HAVE to be...
Anything over PC3200 memory (400mhz) is for overclocking, so look at the timings when u are going to purchase memory ... I didnt and my timings SUCK... 3-4-4-8
Luckily the memory has no problems running at 2.5-3-3-10...Still not awesome but better
Memory timings are especially important with AMD systems, although you might not notice any difference when u do "usual" things with your computer (meaning not gaming, clocking, or benching)
DDR2 memory is just fast memory (high FSB speeds need fast memory) with sh*tty timings,
1t or 2t command rate > 2T command rate is slower, especially on AMD systems its a real performance killer...However when u have more then two memory banks in your Motherboard, 2t command is needed
Be carefull with setting those timings...:
T command rate should be available in your BIOS, memory timings may not be...
If you have an AMD system, u could download the "A64 Tweaker" < google it or something..
Be sure you DONT set that your settings are enabled at bootup!!!Otherwise when u get a black screen, which u probably will get a few times messing with the memory timings, u dont have to switch jumpers on your mobo, or pull out the batteries...U can then simply power down and reboot..
Do some reading first before screwing with those timings