You can get 8GB on the XV6800. You need to download the SDHC.777 drivers. I believe that nemanracing is the person who uploaded those drivers. I can't find them at the moment.
There is also another reported SDHC patch working on XDA-Developers. It is located at
http://forum.xda-developers.com/showthread.php?t=344338
Also, 4GB cards work on the XV6800 out of the box.