Replace your primary PSTN phone line?
VoIP is a good replacement for your second phone line, but it's not yet as reliable as PSTN. So, you can keep your primary phone line or at least a cell phone that you'll have a working phone for emergencies and for when VoIP is not working - such like electricity blackout.
VoIP SoftPhone
Software VoIP is a good for getting started or for testing, but hardware VoIP is much more reliable, and that is what you really need a IP phone or a Wi-Fi PDA
Make sure your DSL Router do work with VoIP
Many problems that seem to be VoIP problems are actually DSL Router/Modem problems. It has found several times that a mysterious VoIP problem will go away when just switch Routers. In general, a cheap home Router (with a simple NAT) is probably going to be better for VoIP than a fancy Router with a symmetric NAT and firewall.
