Video ram cut in half

Well, i just bought the new imac 21.5 inch. And i am so satisfied! i configured it with and 2.8 i7 prosessor and 8 gb ram. It came with a 6770 512mb video card.

But when i installed windows 7 ultimate 64 through bootcamp, and type in dxdiag in the seachbox, it only shows 229 mb of video memory. This is a huge

problem because the only reason i boguht this imac was so i could play games at a relativly fair level of graphic. If i right click on the desktop and click screen resolution it shows 512 mb.

But this website systemerquiermentslab.com shows also 229. So it really is 229, how can i get the full 512?

greetings from cold Norway!