Softphones: X-ten Lite is good and free, X-Pro has few more features.
SJPhone is the next most popular choice.
I get good call quality with X-Lite and X-Pro over 54Mb/s WiFi and using a
QoS enabled router (Draytek 2600VG).
You may have bit more of a problem with 11Mb/s WiFi connection (WiFi works
in simplex mode, i.e. it's either send or receive, not both at the same
time).
QoS allows you to set priority for certain types of packets, in this case
VoIP. It can only be done on QoS enabled a router (not many).
Regards,
Martin