(PS plebs - don't forget to say hello in the 'Who are you' section!!)
I've been looking into doing this for myself, so what I'm suggesting here is theoretical and based on my reading and also a short time playing with mythtv on an old set of hardware!!
You'll need to connect the dreambox via a tv card.
This can be done in one of two ways:
a) using a SCART to SVHS/Sound (there is a set of connections including this type on offer at aldi at themoment for a fiver).
b) using the yellow (picture) red/white (audio) connections
Which is most appropriate depends on the tv card you have in your system. Some digital tv cards do not have SVHS/sound input, in which case you're probably best adding an old (and cheap) analogue tv card to your system specifically for connecting to the dbox (I'm planning on buying a Hauppage dual digital card plus a cheap analogue card i've had lying round for ages for my system). Mythtv will then let you create the input for the extra analogue card as the SVHS connection.
Not sure on automatically changing channels - I think you can get an infra-red gizmo that can learn to control the dreambox (I've seen some settings in the mythtv setup for this type of device), but can't be much help on this as haven't got this far with my research yet - lol!!