7950 gt 512mb &vista=nono

i have a vista machine with 4gb of system ram... a 6000+ amd dual core....dual monitors, 22 and a 15 running at different resolutions and a k9a2 cf motherboard... I am having issues with vista... where my screen will flash randomly... I went into DXDIAG and it says that i have almost 2gb of video ram when this card only has 512 mb of ram.. it says its sharing like 1407 mb of system ram when i go into the adapter properties

main point of this post: how do i turn system ram sharing off in vista?