Q: connecting a pc with Radeon HD 4550 graphic card to Cinema Display 27"

I connect a PC with Radeon HD 4550 graphic card to a Cinema Display LED 27" with
ATLONA AT-DP400 Dual Link DVI to Mini Display Port but Apple monitor only display 1280x resolution as the highest display. Where can I find a display driver to install on my Windows 7 64x machine? Thanks!