The TechZone explains the difference between Memory Bandwidth vs. Latency Timings
With all things equal, a stick of DDR memory capable of running 2-2-2-5 will make the computer operating experience seem faster than a DIMM which may only run at 3-4-4-8. This is because the delay from when the memory receives an instruction retrieves the data, and sends it back out is less.
Where it starts to get confusing is when you have the choice of buying high speed memory with slow timings. Just about every PC3700+ rated memory module we've seen uses conservative timings after all. If your answer would be to buy fast memory with tight timings, I'm afraid you're going to be disappointed as there are no such modules available yet. So, why are we still interested in fast memory with slow timings then? Well, the answer goes something like this.. Read on over at TheTechZone.