>how many clients can i use with one server.
If not modifying the system I think the first barrier you'll run into is the number if IP addresses reserved for MDs by the DHCP server. In reality, it's the performance of your system that will set the limit. 5-7 MD should not be a problem
>can i install 2 dvbS cards for watching all programms in horizontal und vertikal simultan (record and watch one) and which are the best (Technisat SkyStar HD2 - DVB-S2 Karte?)
You can install several TV tuners. They can be used for any mix of recording and viewing.
>can i watch on every client a different satellite program?
Yes, provided you have enough tuners
>can i watch full hd 1080 on clients also?
>is core2duo/4gb ram/enough hdd/1gb lan good for the server?
That should be OK. Remember, the MDs are responsible for much of the media handling, so the work is divided by the various system components.
>info for an cheap client.
I'm using an AsRock 330 ION, works great, also for HD. There can be some lagging/pixelation sometimes, but mostly quite OK. (I didn't to too much VDPAU optimisation yet).
>can i control on an extra sound output a musikplayer? (amp in basemant, aktiv to all rooms, outside, garden etc)
Look in the forum for recommendation about music zones. I use Squeezebox 3, but you could add USB sound cards to either the core or to the MDs. The easiest setup is to add a physical Squeezebox to the room where you want music, or to add USB sound cards to the MDs and run SqueezeSlave in the MDs.