Vista - General Advice

Before overclocking (or, indeed, underclocking) a system you should first of all ensure that it is running stable at the stock frequencies. Unless your system is stable at these settings, it is extremely unlikely that you are going to have any success at other frequencies. You should also ensure that you have adequate cooling in your system, particularly if you are going to overclock.
For further information on how to over/underclock a particular motherboard/memory/processor combination, check on Google to see what other users of the same configuration have achieved. Remember that just because someone has achieved a 20% over/underclock it does NOT mean that you will be able to - a lot depends on the actual components used and some, even from the same production batch, are more tolerant to variations in operating frequency than others.