The ram speed needs to be matched to your cpu's front side bus speed, ideally at 1:1 but using a ratio if not (note current Intel processors at stock speed and multiplier ie. 1066 fsb can't usually be run at 1:1). The auto setting in your BIOS will normally take care of this.
Slowing your ram to slower speeds than the FSB will severely slow your system down as you'll bottleneck it.
If you want to tighten timings, go to the manufacturers web site and look up your ram. Timings should be listed. Set these timings in your bios.
These are usually the guaranteed timings. If you want to try tighter timings, then increase the timing speed in steps of 1 unit and see if your computer a) posts and b) passes a test with memtest 86.
The most important element to try 1st is the jump from 2T to 1T. This can be adjusted before touching any other timings.
If your system passes both tests then those tighter timings are ok. If you want to try further, tighten them further. When your pc no longer posts or passes memtest, or both, thats the time to back the timings off a little until you get successful results in both once more.