Timing and speed of memory

I've read some threads about motherboards for the I7 860 and 920 as well as some amd cpus to. I think on the 860 some issues came up about getting the right timing DDR3 chips. I think someone posted they wish they got 1333 speed because it is the default of the bios on the MB and a higher speed was causing some trouble?

Next to the speed there is a timing which usually looks something like (ex: 5/5/5/5/9) I just made that up but you that know, know what I'm getting at. So beyond knowing I would want ddr3 how to you know what you want to actually buy? For whom does this info matter for what purposes.

thanks for any info