Ram-Problems

[BR791818]

hello
i have a hp/compaq dl 380 g3 with two 2.8ghz xeon cpu's and 5120mb ram. the ram is detected correctly by the hardware. but windows 2000 / windows 2003 only detecs 3,5gb ram.
is this a bug of windows? im using w2k adv. server engl. sp3 or w2k3 enterprise edt. engl.
in my opinion the w2k adv. server should address ram up to 8 gb?! is this a addressing-problem of the cpu's?

thanks for help

christian