Question

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? Otherwise, you'll need to reboot your machine to get into the BIOS to determine if there is interleaved memory support on the mobo.
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.
6. The answer in no. 5 is meant to be more of a "better safe than sorry" answer, rather than a "cargo cult" answer. Why chance it, and with memory prices at reasonable levels at this point in time, why bother? Stick with pairs of matched memory, or if you don't want to buy sets of sticks together, get multiples of two for the same type of stick.