After lots of searching, reading, trying and asking I finally have it up and running. I'll try to share my knowledge here...

For mythtv <0.18 you will need to start at step 1, if you have 0.18+ you can start at step 4...
I used KnoppMyth R5A15.1. (with kernel 2.6.11.7-chw-4)

1. You need the firmware file "sc_main.mc" which you can find in the Windows Driver version. You may need to install the driver to get the file...
2. Copy it to "/usr/lib/hotplug/firmware" (for Kernal 2.6+, the 2.4 kernels use a different directory!)
3. Change right with "chown 644 /usr/lib/hotplug/firmware/sc_main.mc"
In R5A15.1 the firmware is already included and is named "dvb-fe-sp887x.fw"

4. Change "/etc/modules". Add the line "dvb-bt8xx" to the start of the file.
With KnoppMyth <R5A12 you may also need to add the frontend "sp887x" for the card...

5. restart. The driver should get loaded and hotplug should successfully upload the firmware

6. Startup. Quit mythtv. Open a xterm.
7. run "/etc/init.d/mythtv-backend stop" to stop the backend
8. run "mythtv-setup" and configure your card. It should appear in the card configuration. If it doesn't, something went wrong. If it appears, but can't be opened you forgot to stop the backend (s.a.)

If you can't scan for channels (error tuning to transport), try specifying at least the bandwidth, transmission mode, guard interval, hierarchy and constellation parameters. If they are set to AUTO, the card/driver may not be able to scan...

Worked for me. I can watch DVB, use timeshift, etc...

The thing that's left is the remote. You can include the module "ir-kbd-gpio" and use an xmodmap, but it seems easier to follow Simons excellent description of how to make it work using LIRC and "lirc_gpio". Read it here: http://mysettopbox.tv/phpBB2/viewtopic.php?t=5681
Works without problems now with my 761 card!

Greetings

rarefluid