Serg has posted some great information about fret buzz on his site Guitar Daily. He goes into perfect detail on the causes of fret buzz and how they can be rectified.
To sum up his posts causes could be:

Old strings

New strings that have not settled
Action is too low
Bent or warped neck
Inconsistent fret heights

If you are experiencing [...]

As with many things pertaining to guitar, the type and size of strings are all preference. At most guitar stores there will be a selection ranging from $5 strings up to maybe $35 for premium strings. Strings come in all sorts of different thicknesses and are made from different metals, so how do you choose [...]