System Info: Installed Physical Memory?

Anyone familiar with this?

So I went into System info (In Vista Ultimate 64) to just check how everything is going and I noticed this for my RAM:

Total Physical Memory:  8GB

Installed Physical Memory:  4GB

Available Physical Memory: 6.5GB

Can anyone explain why the installed phys. memory is showing 4GB?

I went into BIOS and all sticks are showing up, Welcome Screen is showing 8, system monitor Gadget is also showing 8gb.  Is this just a quirk with SP1? 

TIA!