From the MSI website:
<snip>
Main Memory
• Supports eight memory banks using four184-pin DDR DIMM
• Supports DDR SDRAM DIMM
• Supports a maximum memory size of 4GB (registered DDR only)
Note: Because of limitations imposed by the personal computer architecture and BIOS, the user-accessible memory is less than 4 Gbytes even if 4 Gbytes of physical memory is installed.
</snip>
And their FAQ states this:
<snip>
Due to the limitations imposed by the personal computer architecture and BIOS, the user-accessible memory is about 3.5GB when 4GB memory is installed.
</snip>
Also, do you have the latest bios? (v1.91) I don't think it will help but it does mention something concerning memory.
<snip>
a.Support AMD MP 2800+ in correspondance with AMD web site recommendation.
b.System cannot boot when 4GB memory in system and Video memory is 256MB.
</snip>