It's probably a driver issue with Windows 7 Beta. Check the ATI forums to see what info there is there regarding drivers for Windows 7 Beta. But it's not uncommon for a beta OS to not have full support for such things. NVidia didn't release full driver support for the 8800s for Vista until the day after Vista's retail launch. I could be wrong on this particular issue, but beta OSs are generally not going to give you the best gaming experience.
Arokia, are you really running Build 7000? If you are running anything below 7100 you had best switch out very soon. Starting July 1 of this year, the old betas are timing out. Download the free copy of the Windows 7 RC from Microsoft ( http://tinyurl.com/832nco ) and reinstall.
Beyond that, are you using the Windows 7 specific ATI Catalyst driver package, or are you using the Vista one? If you are using the Windows 7 specific one (WDDM1.1), try switching out to the Vista one (WDDM1.0). You lose a bit of resource usage efficiency with the Vista ones, but you might gain some stability.
Kurtis Whittington - Microsoft Windows Client Team