PowerEdge SC1435 memory issue

Hello,

I currently have 4GB installed in DIMMs 1,2,5,6.

I bought 16GB (DDR2 PC2-5300) additional and tried to install them in DIMMs 3,4,7,8. 

I get the following message at boot: "The current memory configuration is not optimal", however when logging in to Windows 2008 R2 Enterprise I see the 20GB but it says that only 4GB are usable. 

I also tried just the 16GB in DIMMs 1,2,5,6. but the server won't even boot.

I can return the memory, but what is the right type of memory for this server?

Thanks.