the lower the better. but to get lower timing you have to lower the speed of your ram for example 800mhz ram(ddr2-800) at 4-4-4-12 might get 3-3-3-9 at 667mhz
but you might be able to lower some timings at your stock speed if you have good ram.
but if you really dont have a clue what your doing id say dont mess with them untill you do some good reading up.