Internet surfing happens in short bursts. That means that the you go to a page (follow a link or enter a web address), the page (down)loads which is the transfer burst itself, and then you read the contents of the page and then decide what you do next. That means that most of the time the line is free, so you should not feel ill effects in your online gaming experience under normal browsing circumstances.
No it shouldnt slow you down as web pages are small..
Seen Wireless router with erthernet connection and an access point for 100 on Dabs click here
What you do is connect the router to the phoneline connect your comp to the router ether via the ethernet/network card (if dont have one they 10pound max (pci)) the connect the access point to mums comp and you are ready to go!