All summed up..
Buy for the highest bus speed. Timing can be adjusted on your own, and wont bring any noticeable performance. Lower timings don't offer you better overclocking. Decreasing your timings can actually make your RAM less stable.
Example. I have A-DATA 2x1gb sticks.
They came at a stock 5-5-5-18 @ 1.9v, 800mhz
I overclocked them to 5-5-5-18 @ 2.1v, 920mhz
The mhz speed is proportional to your cpu speed. So increasing this increases the clock of your cpu (what your goal is.) I had to increase the voltage to the ram to make sure they keep stable at that speed.
I could decrease my timings to 4-4-4-15 but that wont help me overclock. It would just ever so slightly, increase the performance of my ram. Its not worth the trouble.
So by doing all this I got my e8400 at 4ghz