glReadPixels and correct depth values with FSAA

Is there any way to get the correct values (via glReadPixels) from the depth buffer when FSAA is enabled?

Enabling 2x 4x and 6x FSAA mode on a Radeon 9700 pro produces some noisy depth maps.

Scaling the buffer size consequently doesn’t help anymore.

Is it possible that glPixelTransfer helps?

Thanks!