RAM Problem with Dell PowerEdge SC1420

Hi !

I own a Dell PowerEdge SC1420 which have 10Go of RAM, recognized by the  BIOS and by Windows  64bits.

I find it's slow while performing some works, like for example using Excel with quite big files or using wamp server ...

So I have had a look into ram windows performance monitor, and I saw that while processing intensive work maximum RAM used whas ~4Go ! ...

I made a memtest86+ with USB bootable : Pass Completed - No error.

There are 6 memory cards :

- slot 0 : 2048MB DDR2-666 ECC - Crucial

- slot 1 : 2048MB DDR2-666 ECC - Crucial

- slot 2 : 1024MB DDR2-400 ECC Micron 18HTF12872Y-40EA2

- slot 3 : 2048MB DDR2-666 ECC - Crucial

- slot 4 : 2048MB DDR2-666 ECC - Crucial

- slot 5 : 1024MB DDR2-400 ECC Micron 18HTF12872Y-40EA2

Thanks for your precious help !