Oh you are reffering to a power line adapter
like this one. Then this would connect to an AP. These work by using yor home power wiring like an ethernet cable. The issue becomes line noise and cosequently speed. For example the afore mentioned one is rated for 14 Mbps max. A slow wireless G connnection is still usually good for 48 Mpbs. This might not effect web surfing that much but file transfers (ie streaming music/video) and over the network backups will most likely be slow and chopy.
*Edit* I just noticed Netgear makes some rated for 85 Mbps. It's about $150 and still requires a seperate AP for wireless capability.
It would be simpler to get an AP and set it us as a repeater so all it does is boost the signal and resends it.
Just my 2c but what do I know.