You don't buy them from Sky 't', you purchase them from a company called FreeSat (the number is in telephone numbers option on your sky+ box menu's and sky will allow this obviously - after a year it's your box anyway).
Leaving the card in for atleast 3 days won't 're-activate' it as a freeview card either! - it will give you ch4, 5 and sky3 anyway (even for these aparent 3 days) as you have been a previous subscriber (after 3 days, nothing will change ^^). However, to view aditional channels i.e channel 4+1, Film4, Dave etc you will need a freesat card. Also its about £10 - £15 and the freeview channels are not 'Completely' different, freeview simply has a few more channels you will never watch. Best not to give advice if your not sure..
To enable it to record to Sam - you can organise this through FreeSat I beleive but you need a card in the box as they are paired with each other. i.e if you put your viewing card into your friends sky+ it will not let you record for the same reason. It needs a card and it needs to be paired. I havn't done any of this, I work for sky's repair plan and I was taught it.
Hope this helps someone, I got bored at work and noticed this thread
Inque, September 2009