I am running Ubuntu 9.04 LTSP and have Webmin 1.480 loaded. The server is an HP DL385 x 2 Quad Xeon G5 with 8GB Memory. In Webmin-System Information, the Real Memory is showing up as 3.20 GB instead of 8 GB. From the command line I run the lshw -C MEMORY command and all of the memory shows up correctly.
What does the /proc/meminfo file contain on your system, and what does the free command output?
It may be that the kernel isn't picking up on RAM above 4GB.