I have tried using a CAM and card with a pc tv adapter before, and it's not straightforward. Firstly, such an arrangement is NOT supported by TopUp TV, and so when you phone them to get them to activate the card, you have to lie and say you have one of the approved set top boxes. Unfortunately, there aren't any approved boxes anymore as they (TopUp TV) have moved the system to TopUp TV Anytime, whereby you buy one of their boxes, which downloads certain programs from a list of channels you can select, it does this overnight, supposedly when you aren't using the box. I have experience of this too, and it doesn't work satisfactorily at all, either the downloading or the set top box. The box has a hard disk for recording and is prone to the usual problems when a disk goes bad, which happened within 6 months in my case.
Also, when I say download, this is from the normal dvb signal, not from the internet.
But in any case, unless you can get channel 5 clear as a bell, in any weather or conditions, steer clear, because ALL the encrypted channels are broadcast over the channel 5 multiplex, and the activation won't happen unless you can tune to channel 5 and leave it that way overnight.
In other news, does anybody want to buy a 2nd hand TopUp TV compatible CAM ? It's a Mediaguard CAM and doesn't include the viewing card (I returned that for a refund after the system wouldn't work)
If you want to know more about CAMS and dvb go to :
(You can get a usb cam for about £16)
AFAIK, CAM access is working in linux, but I haven't tried it. See
http://www.ethics-gradient.net/myth/mythdvb3.html for an old howto.
I have 2 Nebula dvb-t pci cards which work fine under linux with MythTV (installed using the MythDora iso found at http://g-ding.tv/?q=MythDora )
Finally, nothing works with sky other than sky equipment. This is by design to prevent fraud, and even the free to view channels require a sky card to decrypt them. I have a totally free sat system (free to air) that requires no card, but as you say does not have some of the normal (free) channels you get with sky. But 30 + channels is usually enough I find. All BBC, all ITV, various film channels, Film 4, Men & Motors etc etc.