EDIT:
Hey guys!
First off I want to thank this whole forum for it's existence! Although it was a pain in the a** (at least for me) to read trough dozens of articles, getting more confused by each, I wouldn't have known what to do without it.
I finally took a maybe very radical but reasonable solution:
Since I didn't know anything about my softmod besides that it was very different from the modern ones, I decided to erase my whole C: drive (after making a backup!) and installed a virgin state original msdash which I was lucky enough to stumple upon the internet.
I then applied the most recent krayzie ndure softmod. I also installed triple boot, meaning that I'm able to switch between original msdash, evox and xbmc.
That all functioning well I made another backup (honestly guys, you can't ever make too many backups!

).
Now comes the difficult and dangerous part. I loaded krayzies extras and it's professional mode. In pro-mode, your whole system is defenseless against changes, so don't screw up!
But that means you're also able to change the original eeprom, which is exactly what I had in mind. So back in evox, pro-mode still active (indicated by your led shining red, which is more than appropriate

) I ran Enigmah video switcher, changed video mode to NTSC, reseted. Now I went to krayzies extras again and updated the eeprom (as I understand, that is necessary because you have to update your virtual eeprom as well). After reboot my msdash appeared in an ugly-as-hell flickering, which let me know: I succeeded!
Next I enabled all HD-modes, plugged in my neoyo X2VGA adapter, started xbmc and switched to 720p resolution.
And it just blew my mind. There is absolutely no comparison to that flickering blurryness I had to endure before that!
There are some lose ends, however. First of all, it seems as if most, but not all PAL-xbox-games are running on NTSC. Why they do while others don't I can't tell. Judging by the appearance I wouldn't say that many of them support high resolutions. And if they were running on 480i it would look like my starting screen with the xbox logo: cruel and flickering. By the way: is it possible to tell msdash to run on at least 480p or even 720p?
Furthermore, the xbox CPU & GPU seem to be to weak to handle mkv video format on highest resolution. Limiting video resolution to 720p and playing movies using dvdplayer instead of mplayer made it better, but not much. There are still lags while playing.
So obviously I still have some reading to do. Anyway, the whole thing was well worth the efford. Thanks again for the help!
Best regards,
FuzzyQ