I followed this tutorial to the point:
 not allowed to link directly, it's the Install_CM_for_ace guide from cyanogenmod, first google hit when c/p into google [/edit]
Welllll NEARLY to the point, with the result of me having screwed up. To be precice, I overlooked the last, really big written Note and did not manually flash the boot image.
I still can boot into recovery mode and in the ClockworkMod Recovery mode.
I can get the device to be visible with ./adb devices by booting into the ClockworkMod Recovery but I can't get fastboot to see it.
When in HBoot menu (of course) not even adb can be made to find the device.
My idea now is to download and install the original ROM (because of course I did ignore the "backup your ****" part because backing up is for pussys and smart people and apparently I don't belong in either group).
I am not sure though which one to downoad and use for this and before screwing up even more, I decided to do the right thing and ask for help
I'm very thankful for every tip!
Thanks a lot.