Apart from the cr*p link I do know what you are on about. Providing you are ok with the low speed (11Mbps) and your two computers are on the same electrical circuit then you should be ok. It's a greeat way for singlies to share an Internet connection.
When you say same circuit, do you mean same fuse ring or same mains supply. One point will be in a house, the other in the garage about 10m away. I believe garage takes power from house, but doubt both are on same ring.
I think you would just need a wireless router and possibly only one wireless card/USB thingy. If your main PC is reasonably modern it will already have an ethernet port on it and you can connect most wireless routers via a bog standard network cable. It would only need to be the one in the garage working wirelessly (is that a word?).
I am more than willing to be corrected if I'm wrong...