Setting up small home network

Ok so this is probably a stupid question but here goes...

I am buying a router with wireless soon and i am setting up an ADSL account.
I only have one computer that will connect to this router.
The router will be based in the living room area where the phone line is.
My room (where the computer is situated) is quite far from the living room and i am not keen to drilling holes through roofs to wire some LAN cable.

My plan is to use a wireless dongle in my pc to connect to the router for internet.

Question: Will this affect the speed of my downloads off the internet at all? will it be the same as if i was using a lan cable?
I know file sharing over the network via WAN is slow but will my internet be?