changing to 802.11g (54Mbps) won't affect any of your other
wireless devices as long as they're not 802.11b only.
changing to 802.11g will prevent any 802.11b (11Mbps) wifi
devices connecting to your network.
to change to 802.11g, you normally have an option to alter the
broadcast mode. it could be either 802.11b only, 802.11g only or
"mixed mode" which allows for both protocols to connect.
with regards to getting your Wii to connect, are you using
MAC filtering as well as WEP or WPA encryption?
if you are, you will need to add the Wii's MAC address to
the list of "trusted" devices which are allowed to connect
to your router.