Jepp, same problem here. In short: it even looks like only half of the ram really is used. To fix this, for fglrx to use all of it with this option in your /ect/X11/xorg.conf:
I just heard it yesterday night from d2kx that there seems to be this problem...
Identifier "ATI Graphics Adapter"
# otherwise not all vram is found/used...
Option "MaxGARTSize" "512"
After adding this, Xorg.log.0 does report that it is using all 512MB vram.