Assuming you're talking about DDR2, first off, the RAM speed, as in DDR800, DDR1000, etc.
The higher the better, and usually, the more expensive it is.
Timings indicate the length of the delay when the memory performs different tasks, such as reading and writing. The smaller these timings are, the better.
The 5-5-5-15 that you are talking about is:
tCAS-tRCD-tRP-tRAS
The most important is the first one, the CAS timing. The others are not as significant to performance (but they do add bragging rights).
If you go from DDR533 and up, you will notice that while the RAM speeds go up, so do the latencies. The performance you gain from the speed outweighs the cons of the higher timings.