There is no Radeon CrossFire X1900 XTX card but ATI asserts that the X1900 XTX will not clock down to match a CrossFire X1900XT Card. Note that this does not necessarily mean that the CrossFire card will automatically overclock. Both ATI and NVIDIA seem to have worked something out with regards to their dual card solutions that allow for asynchronous clockspeeds on the two different cards (NVIDIA enabled this a few driver revisions ago). The X1900 CrossFire will allow for resolutions up to 2560x1600 @ 72Hz.