How would a wireless network which is getting signal from my wired LAN affect my computer/internet connection?

My next door neighbour would like to create a wireless network in order to access internet- through my wired LAN (DSL modem). How would that affect my computer? Could I be having difficulties when trying to access internet at the same time and -would it slow down my connection- speed?How would a wireless network which is getting signal from my wired LAN affect my computer/internet connection?
Sharing an Internet connection is typically not a big deal as long as both parties using it are responsible and considerate -- if your neighbor was just surfing the web, using e-mail, etc you would probably not know he was using your Internet. However, if he was downloading HUGE movies or using Peer to Peer software (bittorrent) it could drag your Internet to its knees. Also, its important to realize that the person whose name the Internet account is in will be held responsible for abuse -- make sure he's someone thats not going to be hacking, downloading kiddie porn, etc because the feds will come after you.How would a wireless network which is getting signal from my wired LAN affect my computer/internet connection?
Yes it could slow you down greatly.