Xbox 360: How much memory for the GPU?

How much memory can*the GPU*use on the Xbox 360? I have seen that the GPU has access to the build-in system memory (which is 512 MB), but can it use the whole amount of memory that's found there or only a subset of it?

I'm wondering that because usually textures use a lot of memory (OK you can compress them but still) and in a game like GTA 4 there are a lot of visible textures.*Now if the GPU can only use like 32 or 64*MB of the whole memory I can estimate who the textures in GTA 4 must look like when you move very near to them.