Those will work fine if you just want to connect one computer to another or just go from the modem to one computer.
However in order for you to both share an internet connection (and be on the internet at the same time), you will also need a router.
I presume you are on an virginmedia connection.
you can pick up a wired router for around £30.00.
eg, this one:
http://www.amazon.co.uk/Linksys-Ethe...sbs_ce_title_4
This is how your set up would be:
Virginmedia modem ------> Router
From the router two connections, 1 going to your computer upstairs, the other one going from the router into one of the homeplugs devices.
Then plug your other homeplug device anywhere on your house's main electricity ring.