Lily,
Firstly you have to think I am with blueyonder (Cable DSL) not BT (ADSL) so the 2nd phone line means diddly squat.
I guess upping the bandwidth would simply mean I have more to use
but if you went down the ADSL route and BT enable a ADSL line to my house you could hang a wireless router on there and just use a different channel, set up access control on the router etc no problem.