Ok, I played through the first few hours and started to have some fun with the game regardless of the flaws compared to Gothic 1+2.
After a while, Bluescreens mentioning a problem with my graphics drivers (Radeon HD5670) started happening 3 out of 4 times when trying to load (any) savestate. Later on random crashes (without bluescreen) were added. And even when the game ran good, I got the Bluescreen after quitting the game.

Changing to the latest driver didn't change anything, fiddling around with the graphics settings didn't change anything either (disabling all Antialiasing, decreasing shader quality, etc...).
When it runs, it runs quite well, but there's no way I could finish this game that way.

Is it simply not playable with Radeon cards in GoGs current version or can I do something else than just deinstalling and deleting all those hours of gameplay with my savestate?

My system is WinXP 32bit SP3, 4gb ram, Radeon HD5670, current patches, dx9, current catalyst driver.

Please help me if possible :(

You could try updating to Gothic 3 Patch 1.75.14 (GOG has an older patch).

Failing that you could try deleting the shader cache in %USERPROFILE%\My Documents\gothic3\

NooooooOOOOOOOoooo I will try. If that doesn't work, I will try the patch and look for a way to retrigger the bluescreens earlier in the game without wasting as much time again.

Which of the files is the shader cache?
Besides the quicksave and UserOptions.ini I got:
G3_World_01_0001.g3savcpx
G3_World_01_0001.g3savcpxdat
G3_World_01_0002.g3savcpx
G3_World_01_0002.g3savcpxdat

Those look like my other two savegames though.
I thought shader cache was introduced in version 1.75.

You're right, I forgot the shader cache was only in 1.75 and later.

I installed 1.75 now and although most stuff seems to have improved, the game runs in slow motion now. oO
I've never seen something like this, everything looks great (FSAA finally works) and no bluescreen or flickering yet, but no matter what I do, the game does have about 1/4 speed, even when setting all graphics settings to the far left.

Swinging the sword takes about 10 seconds.

Edit: I have found a solution for SLOW MO problem on some forum.
Go to "Ini" folder, find 'ge3.ini' file, then find parameter 'Timer.ThreadSafe' set it from 'true' to 'false'.
It worked so far. Let's see if the bluescreens come back.

Edit 2: The combination of the new patch and the ini-edit seems to have solved all my technical problems.
The game even runs and looks better than before and the loading time is reduced drastically.

It's just like with Gothic 1 back then, although the patches over the years that finally made it playable were not fanmade but technical support from the developers if you ignore purely cosmetical stuff like the HQ-textures and the widescreen patch.

Edit: Later in the game, the bluescreens came back, no way to make this game playable with 4gb RAM (WinXP maximum).