Humble is pretty much right. Those numbers deal with memory latency and how long the computer waits for the memory to sort out an answer after getting a query. Memory isn't instantaneous so if you measure the response from a memory stick when you give it an address, the initial response may be just random garbage. So the motherboard has to wait a certain amount of time. And then once the answer is there, the quicker it's read the quicker it can go to the next address, so you don't want the answer to be available longer than necessary to be read. Those latency times adjust the wait time before it's read, how long to expect the answer to be available, and the delays before the next address is requested.
The first number is the typical "CAS level" you hear about and is often advertised as the memory speed. Cas 2 is usually the best, although there may be CAS 2 memory rated at 2-3-3-8 and another cas 2 stick rated at 2-2-2-6. The main thing to remember is that in general, the lower the number the better. Really high speed memory might typically be rated 2-2-2-6. Some overclockers have noted that some memory runs faster with the last number set at a specific value, not just the lowest number. I've read that the crucial chips in my computer run best at 2-2-2-10, but people have had good results with the last number anywhere from 8 to 11.
In any case, probably more important than that is the command rate. It's usually either 1T or 2T, and 1T is faster. Some people claim that it makes a huge difference, but basically they all affect memory response latency and bandwidth so any one setting by itself shouldn't kill or add tons of performance. Most athlon 64 systems produced up to now have had serious problems running 1T command rate when there are 4 sticks of memory installed, so you just have to increase it to 2T to get it to work right. I've read that this issue is actually a problem with the cpu not the mobo or chipset, and that AMD has sorted it out with their latest cpu revisions.
2.5-3-3-8 is just fine for normal memory. If you want to push it lower, go ahead but test the crap out of it using memtest86 before you boot into windows otherwise you risk hard drive corruption. Memory errors often don't immediately crash your system but they'll cause random errors and eventually that will kill your operating system or just make things not work right.
Sucky memory will be rated at like 3 5 5 12 or worse, so cas 2.5 is just fine. Since you're running a single channel memory controller, you may get higher performance gains from fast memory than someone on a dual channel controller, because you're on half the bandwidth so latency on that one channel is a bigger deal. I wouldn't spend hundreds of extra dollars on faster memory though, especially if you got a socket 754 mobo just to save $30 or so.
As a final note, many motherboards have preset options for memory latency. "normal" might be 3-4-4-10, "fast" might be 2.5-3-3-10, and "turbo" might be something like 2-3-3-8. The way to check is to get a utility like cpuz because it can check to see what the memory is running at. I think memtest86 also reports memory settings.
