1. Keep your existing memory.  You may need it during the lifespan of the machine.
 2. On SMP-style machines, especially the ProLiant boxes, there are diagrams of the memory slots on the internal side of the lift-off cover.  Usually there is a suggestion there if memory interleaving is needed
 3. While you can get away with non-interleaved style installation, I usually stick with it just for symmetry.  *It also allows you to set any BIOS settings to enable interleaved memory access.*
 4. You did check your BIOS settings before placing it into production, and recorded their settings, didn't you?
 5. Try to buy your memory in pairs.  Just trust me on this - you'll have less grief when you have paired memory, especially in a server that has separate memory banks that are unified by the mobo's chipset.