The ideal case for you is to use a dvb-s/dvb-s2 card, a card reader and a YES smart card. you will not need an STB (memir) for this, and you get the digital signal with no quality loss. you will also probably be able to watch/record multiple programs with one dvb-s card if they share frequencies (both myth and vdr support that).
i am not a YES subscriber, and i have never done that with YES, but i think i read about people which have done this. I don't know if there are legal issues here. but as i see it, you are not stealing if you have legit smart cards from YES. just beware that sattelite HD (dvb-s2) is in it's infancy under linux. I think it is was added to linux starting from 2.6.18 (or you'll need to patch the kernel) you will also need to the svn version of mythtv/vdr/whatever for dvb-s2. (political issues and a lot of ego posponed linux support of dvb-s2, about a year ...) I use dvb-s (SD) for few years now. works great (but not watching YES). i'm waiting there will be enough HD content, and better linux support before i buy an HD card (dvb-s2). cheers, erez.
