Thank you for your reply. I know that the same memory modules are important.
And yes they are all from the same manufacturer and model.
My problem occurred because the rather old Linux 2.6.x-EM64T-P4 SMP kernel did not recognise more then 2 GB. Last night, i installed the 2.6.26-1-AMD64 kernel from Debian and now all memory (8GB) can be used.
The reason i still had an EM64T kernel was from the period that Debian AMD64 was unofficial. I was still using it, because it is very stable and i had never problems with it. From now on i will use AMD64 which comes with Lenny.