I'm not sure this will work all the time though at the size you set it seemed at least on my screen to be centered though if you go bigger the height may become a problem with centering vertically.
Try
Global __winSizeX = 1024
Global __winSizeY = 768
Global __winFPS = 60
SetVirtualResolution ( __winSizeX, __winSizeY )
x = GetVirtualWidth ( ) : y = GetVirtualHeight ( )
SetWindowSize ( x, y, 0 )
SetWindowPosition ( ( GetMaxDeviceWidth ( ) - GetDeviceWidth ( ) ) / 2, ( GetMaxDeviceHeight ( ) - GetDeviceHeight ( ) ) / 2 )
SetSyncRate ( __winFPS, 0 )
Again at your dimensions it seems centered but when I set it to 1920,1080 then the vertical centering doesn't work. So it may not work all the time.