If you just want to be able to see the same thing then you just need to run an ariel from the back of the sky box to the TV upstairs.
If you also want to be able to control it from upstairs then you will also need a sky link (
http://www.amazon.co.uk/SKY-TV-LINK/dp/B0013JB236).
If you want to be able to watch seperate channels then you have no choice but to go for multiroom