RAM Upgrade Recommendations for R620

Hi,

I've got a dual processor R620 server with 8x 8 GB RAM modules giving a total of 64 GB. I now want to upgrade the memory and have seen that the best module to buy (GB/cost) is the 16 GB part.

Can someone tell me what options I have?

Can I keep the existing 8 GB modules in the server and add in additional 16 GB modules? If so, what combinations can I add? 

For example, can I only add 4x 16GB, 8x 16GB 12x 16GB or can I get away with other combinations?

Thanks for looking :)