There is no new firmware for the 6400 series PVR's that will automatically recognize a larger HD than 160gb's, but there is a place that sells "PRE- Formatted" HD's that apparently will work with these PVR's
The Place is called HDTV sales »
www.hdtvsales.ca/ and they sell a 500gb and a 1Tb drive. Please note that you need a special screwdriver to be able to do the swap and I believe they also carry this tool.