I have a W7 64 bit PC with an ATI Radeon HD5570 video card. I have an older Samsung display (Syncmaster 2033SW) connected to the video card via DVI and a plasma screen connected through the cards HDMI.
The max resolution of the pc's screen is 1600x900@60 HZ and the max resolution of my plasma is 1080P.
How do I configure W7 to set these to be the same, or how to be most compatible?
To make them the same you could downgrade the plasma to 1600x900, but why? What exactly are you trying to do?
When I have two dissimilar displays, I set each to its native resolution unless I want to show the same image on both (for example, putting my desktop session up on a projector in a conference room).
If you are going to expand your desktop to just have two monitors, with different content on each, use the native modes. You can use Desktop Properties to adjust the relative positions of the screens so that as you scroll to the right off the left-hand screen the cursor scrolls onto the right-hand screen.
If this isn't good enough, could you give a little more detail about how you want to use the two screens?
EDIT - in case the question was simply how to adjust this, right-click on an empty piece of screen, select Properties, and go to the Settings screen.