Basically the higher the better. There is no ideal but you need a minimum of 10db to avoid connection problems. Mine was 28 on fixed 2 Meg but I'm on MaxDSl now which is a whole different kettle of fish!
If you find yours is near to 10 there are a few things you can do to try and increase it, i.e. if you connect through an extension, try connecting through the master socket or even better the test socket under the masters faceplate. Sometimes doing this can see an increase in noise margin.
Also, check to see if the ring wire is connected to terminal 3 of your sockets (or indeed any other wires besides 2 and 5), this can reduce interference and again result in an increased noise margin.
If you connect through a telephone extension cable, bin it and substitute it for a Cat5 network cable.