No I changed the GT520 to an SAPPHIRE VAPOR-X HD 4890 2GB.. the System is booting up, but hangs on a white screen exactly before the login screen appears..
and the boodloader is flashing something like: Bad display config Block signature for a second or so, before switching to the apple logo with the spinning wheel...
also tried to boot -v -x
last line is:
DSMOS has arrived //but nothing is coming up
and I also see a line:
NVDAGF100HAL loaded and registered. //isn't this an Nvidia Driver thing ?