I have a 5970 and software like GPU-Z show it as a 2GB card,whereas windows DXdiag tool reports the approximate total memory of 3807MB(because of shared video memory) , i remember that with my older cards it was the same(DXdiag always showed more video RAM)
I don't think you can disable it(the shared video memory),unless you have an integrated card which can be disabled from the BIOS