One fahrad will do, two will do & so will 20. One will improve your system quite a bit, two will improve it quite a bit more, three will improve it a little bit more, four will probably not be noticable.
Its works kinda like ram in a computer, going from 32MB to 64MB is quite an improve, going from 64MB to 128MB same thing, going from 128MB to 256MB will make quite a bit of difference, from 256MB to 512MB some difference, from 512MB to 1GB will make a slight difference & so on.
There will be difference between 2 & 3 fahrad. IMO, you should get the 3 to maximize the gains.