GTA IV

Hi, when i try to put my settings on GTA IV al high...i just cant do it! i can move the button for the resolution but it stays on 800x600 and i want to play at 1920x1080...also the details are all on low...down there it says that my system is not good enough to play that game and that im near the limit of my system.
I think thats weird because i have a intel core i7 2600k @ 3.4 ghz and a HD6990...the graphics memory usage in on 300mb while my card has 4000mb of graphics ram...and down in the corner of the screen it says that my system is on his limits at 800x600....i dont know whats wrong... Can you help me?

My specs are ASUS p8p67 pro rev 3.0. intel core i7 2600k@3.4ghz,8 gb ram ddr3 1600mhz kingston hyperX, AMD Radeon HD6990, Corsair HX 1000W PSU. Im running Windows 7 Professional 64-Bit.