Typically, RAM speeds give very little increase in performance. Plus it can only go as fast as your motherboard can handle. Assuming 800MHz is supported, I wouldn't bother going to it unless the price different is very minimal between 667 and 800. I'd also take quantity/quality over MHz.
As for timing, I suppose the most simple way to put it is "the time between start to finish". Basically, the lower numbers are better. Just look at the recomended voltage and confirm it's not been "factory" bumped past the default for whatever type of RAM you're looking into.