how to upgrade server hp prolient ml350 for 8gb ram

[server upgradation]

i want to upgrade my server hp Prolient ml 350 xeon 3.0 G hz it has got 6 X DIMM slots initially it was working 512mbX4 =2Gb of Ram , kindly advice how to upgrade to achieve better services i am running a network of appox 150 client i have purchased 4GB reg pc2-3200 2x2Gb all HP ram which part no 343057-b21 when i put this ram in reamining 2 slots my server is running but in property of my computer it showing only 3.5 gb ram kindly help me

[Rancher]

You don't say what operating system you are running, but you might check this out: http://support.microsoft.com/kb/283037