512mb or 1gb video card problem?

Okay so hes the problem,i have been getting these problems like my video card has 1gb or 512mb VRAM.On Steam it shows 1024 VRAM and on SystemRequirementsLab also 1Gb,My ATI Catalyst control center,Information center --->Graphics hardware shows me my memory size 1024mb.On the other hand S.T.A.L.K.E.R shadow of chernobyl game install shows me 512mb and a Program called GPU-Z the same thing-512mb?What is the problem?Is my graphics card 512mb or 1gb VRAM.Also i almost forgot the graphics card is ATI Radeon HD 4600series-4670 i think.Thanks.