Hauppauge do a nice range of TV cards. this will come with the software you need too.

For capturing sky of any decent quality you want to use the Svideo port on the TV card. Its doubtful your sky box has an svideo out so you would need an RGB To Svideo converter box. You basically take the scart lead from your sky box and plug into this little converter box. Then that gives you an output to svideo which you connect to your tv card - this is what I do to watch NTL cable on my Media Center PC.

You can get the converter boxes on ebay for about £30.