I tried using a no-name SD card with the default debian6-19-04-2012 image. Not surprisingly the whole RPi system crashed after a couple of lines of error messages on the screen. Overwiting the firmware files in the 1st FAT formatted partition with the updated ones from hexxeh at github made the card work well and I can now log in. Success..... but not for long. When I type 'Startx' to go to the GUI system, it crashes after loading. Reading around the forums suggests that this is because of version mismatches with different parts of the software on the SD card.
I use a Windows 7 PC to write the downloaded images to the SD card, so with Windows I cannot do anything with the linux formatted partitions on the card. If I update the firmware files as I have described above, how do I reliably update the rest of the card so that other software including the GUI system works correctly?