from speaking to staffers (i live in the DC area). They are generally young 20-somethings whose job it is to answer the phones, tally the e-mails & letters, and report on it to the congresscritters; they are generally unanimous on a few points:

1) that the congresspeople care and notice if an issue is getting a lot of attention from constituents 2) that this is something they usually ask about every day 3) that phone calls are generally more influential than letters, which are generally more influential than e-mails -- but it depends on how each office handles their inbounds.