PowerEdge R610 memory

I have a PowerEdge R610 server that currently has 8 GB of RAM. It has 4 2GB chips (2 for each processor, both are using slots 2 and 4). I want to upgrade it to 24GB of RAM and use the server for VMWare. It looks like I can either install the RAM in pairs or in a set of 3. What would be best to get? A set of 2 or 3 sticks? What slots should I use when I install the RAM? I'm thinking dual-rank will be best but I'm not completely sure of that either. Suggestions?